Light-Emitting Element, Light-Emitting Device, Electronic Device, and Lighting Device

ABSTRACT

Provided is a novel light-emitting element, a light-emitting element with a long lifetime, or a light-emitting element with high emission efficiency. The light-emitting element includes an EL layer between a pair of electrodes. The EL layer includes at least a light-emitting layer containing a fluorescent substance and a host material, a first electron-transport layer containing a first electron-transport material, and a second electron-transport layer containing a second electron-transport material, which are in contact with each other and in this order. The LUMO level of each of the host material and the second electron-transport material is higher than the LUMO level of the first electron-transport material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement, a display module, a lighting module, a display device, alight-emitting device, an electronic device, and a lighting device inwhich the organic compound is used. Note that one embodiment of thepresent invention is not limited to the above technical field. Thetechnical field of one embodiment of the invention disclosed in thisspecification and the like relates to an object, a method, or amanufacturing method. In addition, one embodiment of the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. Specifically, examples of the technical field of oneembodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a liquid crystaldisplay device, a light-emitting device, a lighting device, a powerstorage device, a memory device, an imaging device, a method of drivingany of them, and a method of manufacturing any of them.

2. Description of the Related Art

Light-emitting elements (organic EL elements) including organiccompounds and utilizing electroluminescence (EL) have been put to morepractical use. In the basic structure of such a light-emitting element,an organic compound layer containing a light-emitting substance (an ELlayer) is provided between a pair of electrodes. By voltage applicationto this element, light emission from the light-emitting substance can beobtained.

Since such light-emitting elements are of self-light-emitting type,light-emitting elements have advantages over liquid crystal displayswhen used as pixels of a display in that visibility of pixels is highand backlight is not required. Thus, light-emitting elements aresuitable as flat panel display elements. A display including such alight-emitting element is also highly advantageous in that it can bethin and lightweight. Besides, very high speed response is one of thefeatures of such an element.

Since light-emitting layers of such light-emitting elements can besuccessively formed two-dimensionally, planar light emission can beachieved. This feature is difficult to realize with point light sourcestypified by incandescent lamps and LEDs or linear light sources typifiedby fluorescent lamps. Thus, light-emitting elements also have greatpotential as planar light sources, which can be applied to lightingdevices and the like.

Displays or lighting devices including light-emitting elements can besuitably used for a variety of electronic devices as described above,and research and development of light-emitting elements have progressedfor higher efficiency or longer lifetimes.

Patent Document 1 discloses a light-emitting element that achieves along lifetime by including an electron-transport layer to which asubstance having an electron-trapping property is added.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2009-177157

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel light-emitting element. Another object of one embodiment of thepresent invention is to provide a light-emitting element with a longlifetime. Another object of one embodiment of the present invention isto provide a light-emitting element with high emission efficiency.

Another object of one embodiment of the present invention is to providea highly reliable light-emitting device, a highly reliable electronicdevice, and a highly reliable display device. Another object of oneembodiment of the present invention is to provide a light-emittingdevice, an electronic device, and a display device each with low powerconsumption.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

A light-emitting element of one embodiment of the present inventionincludes an anode, a cathode, and an EL layer between the anode and thecathode. The EL layer includes a light-emitting layer, a firstelectron-transport layer, and a second electron-transport layer. Thefirst electron-transport layer is between the light-emitting layer andthe second electron-transport layer. The light-emitting layer has aregion in contact with the first electron-transport layer. The secondelectron-transport layer has a region in contact with the firstelectron-transport layer. The light-emitting layer includes afluorescent substance and a host material. The first electron-transportlayer includes a first material. The second electron-transport layerincludes a second material. The LUMO level of the host material ishigher than the LUMO level of the first material. The LUMO level of thesecond material is higher than the LUMO level of the first material. Thehost material is a substance including a condensed aromatic ringskeleton including 3 to 6 rings. The first material is a substanceincluding a first heteroaromatic ring skeleton. The second material is asubstance including a second heteroaromatic ring skeleton. The substanceincluding the first heteroaromatic ring skeleton is different from thesubstance including the second heteroaromatic ring skeleton.

Another embodiment of the present invention is a light-emitting elementwith the above structure, in which each of the substance including thefirst heteroaromatic ring skeleton and the substance including thesecond heteroaromatic ring skeleton is a substance including asix-membered nitrogen-containing heteroaromatic ring skeleton.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the substance including thefirst heteroaromatic ring skeleton is a substance including a condensedheteroaromatic ring skeleton.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the substance including thefirst heteroaromatic ring skeleton is a substance including a condensedheteroaromatic ring skeleton including a diazine skeleton or a triazineskeleton.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the substance including thefirst heteroaromatic ring skeleton is a substance including a pyrazineskeleton or a pyrimidine skeleton.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the substance including thefirst heteroaromatic ring skeleton is a substance including adibenzoquinoxaline skeleton.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the host material is asubstance including an anthracene skeleton.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the second electron-transportlayer is in contact with the cathode.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the EL layer further includesa hole-injection layer, the hole-injection layer is in contact with theanode, and the hole-injection layer includes an organic acceptormaterial.

Another embodiment of the present invention is a light-emitting elementwith the above structure, in which the organic acceptor material is2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene.

Another embodiment of the present invention is a light-emitting elementwith any of the above structures, in which the fluorescent substanceexhibits blue light.

Another embodiment of the present invention is a light-emitting deviceincluding the light-emitting element with any of the above structuresand a transistor or a substrate.

Another embodiment of the present invention is an electronic deviceincluding the above light-emitting device and at least one of a sensor,an operation button, a speaker, and a microphone.

Another embodiment of the present invention is a lighting deviceincluding the above light-emitting device and a housing.

Note that the light-emitting device in this specification includes, inits category, an image display device with a light-emitting element. Thelight-emitting device may be included in a module in which alight-emitting element is provided with a connector such as ananisotropic conductive film or a tape carrier package (TCP), a module inwhich a printed wiring board is provided at the end of a TCP, or amodule in which an integrated circuit (IC) is directly mounted on alight-emitting element by a chip on glass (COG) method. Thelight-emitting device may be included in lighting equipment or the like.

In one embodiment of the present invention, a novel light-emittingelement can be provided. In another embodiment of the present invention,a light-emitting element with a long lifetime can be provided. Inanother object of one embodiment of the present invention, alight-emitting element with high emission efficiency can be provided.

In another embodiment of the present invention, a highly reliablelight-emitting device, a highly reliable electronic device, and a highlyreliable display device can be provided. In another embodiment of thepresent invention, a light-emitting device, an electronic device, and adisplay device each with low power consumption can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the objects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are schematic diagrams of light-emitting elements.

FIGS. 2A and 2B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 3A and 3B are conceptual diagrams of an active matrixlight-emitting device.

FIG. 4 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 5A and 5B are conceptual diagrams of a passive matrixlight-emitting device.

FIGS. 6A and 6B illustrate a lighting device.

FIGS. 7A to 7D illustrate electronic devices.

FIG. 8 illustrates a light source device.

FIG. 9 illustrates a lighting device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates in-vehicle display devices and lighting devices.

FIGS. 12A to 12C illustrate an electronic device.

FIGS. 13A to 13C illustrate an electronic device.

FIG. 14 shows luminance-current density characteristics of alight-emitting element 1 and a comparative light-emitting element 1.

FIG. 15 shows current efficiency-luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 1.

FIG. 16 shows luminance-voltage characteristics of the light-emittingelement 1 and the comparative light-emitting element 1.

FIG. 17 shows current-voltage characteristics of the light-emittingelement 1 and the comparative light-emitting element 1.

FIG. 18 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 1 and the comparative light-emitting element1.

FIG. 19 shows emission spectra of the light-emitting element 1 and thecomparative light-emitting element 1.

FIG. 20 shows characteristics of normalized luminance change with timeof the light-emitting element 1 and the comparative light-emittingelement 1.

FIG. 21 shows luminance-current density characteristics of alight-emitting element 2 and a comparative light-emitting element 2.

FIG. 22 shows current efficiency-luminance characteristics of thelight-emitting element 2 and the comparative light-emitting element 2.

FIG. 23 shows luminance-voltage characteristics of the light-emittingelement 2 and the comparative light-emitting element 2.

FIG. 24 shows current-voltage characteristics of the light-emittingelement 2 and the comparative light-emitting element 2.

FIG. 25 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 2 and the comparative light-emitting element2.

FIG. 26 shows emission spectra of the light-emitting element 2 and thecomparative light-emitting element 2.

FIG. 27 shows characteristics of normalized luminance change with timeof the light-emitting element 2 and the comparative light-emittingelement 2.

FIG. 28 shows luminance-current density characteristics of alight-emitting element 3 and a comparative light-emitting element 3.

FIG. 29 shows current efficiency-luminance characteristics of thelight-emitting element 3 and the comparative light-emitting element 3.

FIG. 30 shows luminance-voltage characteristics of the light-emittingelement 3 and the comparative light-emitting element 3.

FIG. 31 shows current-voltage characteristics of the light-emittingelement 3 and the comparative light-emitting element 3.

FIG. 32 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 3 and the comparative light-emitting element3.

FIG. 33 shows emission spectra of the light-emitting element 3 and thecomparative light-emitting element 3.

FIG. 34 shows characteristics of normalized luminance change with timeof the light-emitting element 3 and the comparative light-emittingelement 3.

FIG. 35 shows luminance-current density characteristics of alight-emitting element 4 and a comparative light-emitting element 4.

FIG. 36 shows current efficiency-luminance characteristics of thelight-emitting element 4 and the comparative light-emitting element 4.

FIG. 37 shows luminance-voltage characteristics of the light-emittingelement 4 and the comparative light-emitting element 4.

FIG. 38 shows current-voltage characteristics of the light-emittingelement 4 and the comparative light-emitting element 4.

FIG. 39 shows external quantum efficiency-luminance characteristics ofthe light-emitting element 4 and the comparative light-emitting element4.

FIG. 40 shows emission spectra of the light-emitting element 4 and thecomparative light-emitting element 4.

FIG. 41 shows characteristics of normalized luminance change with timeof the light-emitting element 4 and the comparative light-emittingelement 4.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. Note that the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details can be variously modifiedwithout departing from the spirit and scope of the present invention.Accordingly, the present invention should not be interpreted as beinglimited to the content of the embodiments below.

Embodiment 1

FIG. 1A illustrates a light-emitting element of one embodiment of thepresent invention. The light-emitting element of one embodiment of thepresent invention includes at least an anode 101, a cathode 102, and anEL layer 103. The EL layer 103 includes at least a light-emitting layer113, a first electron-transport layer 114-1, and a secondelectron-transport layer 114-2.

The light-emitting layer 113 contains a host material and a fluorescentsubstance. By application of voltage to the light-emitting element forletting current flow, light from the fluorescent substance can beobtained.

The host material is a substance including a condensed aromatic ringskeleton including 3 to 6 rings. A material for the firstelectron-transport layer 114-1 and a material for the secondelectron-transport layer 114-2 are substances each including aheteroaromatic ring skeleton. The material for the firstelectron-transport layer 114-1 is different from the material for thesecond electron-transport layer 114-2.

In the light-emitting element of one embodiment of the presentinvention, the LUMO level of each of the host material and the materialfor the second electron-transport layer 114-2 is higher (shallower) thanthe LUMO level of the material for the first electron-transport layer114-1. Note that a difference between the LUMO levels of the hostmaterial and the material for the first electron-transport layer 114-1is preferably smaller than or equal to 0.3 eV, in which case an increasein driving voltage can be suppressed.

In general light-emitting element design, the LUMO levels of layers onthe electron-transport layer side (layers in an EL layer between alight-emitting layer and a cathode) are designed to become higher(shallower) from the layer on the cathode side in order to reduce thecarrier injection barriers between the layers, decrease the drivingvoltage, and improve the lifetime.

However, the light-emitting element of one embodiment of the presentinvention in which the LUMO levels of the host material and thematerials for the electron-transport layers have the above relation andmaterials including particular skeletons are used can have a longerlifetime than a light-emitting element with the conventional structure.

The host material preferably includes a condensed aromatic ring skeletonincluding 3 to 6 rings because these condensed aromatic rings canmaintain an energy gap near the visible light region and haveelectrochemical stability. In particular, an anthracene skeleton ispreferred because an energy gap large enough to excite a bluefluorescent material can be obtained and holes and electrons both can betransported. In addition, the LUMO level of an anthracene derivative canbe easily set to approximately −2.7 eV, which is suitable for formingthe relation of the LUMO levels.

The material for the first electron-transport layer 114-1 and thematerial for the second electron-transport layer 114-2 preferablyinclude different heteroaromatic ring skeletons, in which case the LUMOlevel of the material for the first electron-transport layer 114-1 canbe lower (deeper) than the LUMO level of each of the host material andthe material for the second electron-transport layer 114-2.

In the case where the EL layer 103 further includes a hole-injectionlayer 111 and an organic acceptor material is used for thehole-injection layer 111, not only the lifetime can be longer than theconventional lifetime, but also a decrease in efficiency in a highluminance region, what is called roll-off, can be suppressed; thus, alight-emitting element with high luminance and high efficiency can befabricated. Therefore, in the light-emitting element of one embodimentof the present invention, the EL layer 103 further includes thehole-injection layer 111, and an organic acceptor material is used forthe hole-injection layer 111.

One of the reasons for this is probably that the above-mentionedstructure can complement the low hole-injection capability of an organicacceptor. The acceptor properties of many organic acceptors with respectto a material having a HOMO level lower (deeper) than −5.4 eV are low,which means that it is hard to inject holes. For this reason, in thecase where an organic acceptor is used for the hole-injection layer, amaterial having a HOMO level higher than or equal to −5.4 eV ispreferably used for the hole-transport layer, on the other hand, it isdifficult to inject holes from such a hole-transport layer to thelight-emitting layer. This is because the material including a condensedaromatic ring skeleton including 3 to 6 rings often has a HOMO levellower (deeper) than −5.4 eV. Accordingly, in the case where an organicacceptor is used for the hole-injection layer, a barrier ultimatelyexists when holes are injected to the light-emitting layer, and thus theorganic acceptor has low hole-injection capability.

Consequently, for example, in the case where the firstelectron-transport layer 114-1 is not provided or in the case where theLUMO level of the material for the first electron-transport layer 114-1is similar to the LUMO level of each of the host material and thematerial for the second electron-transport layer 114-2, thehole-injection property and the hole-transport property of thehole-injection layer for which an organic acceptor is used are low ascompared with the electron-injection property and the electron-transportproperty. In particular, when the element emits light with highluminance, the element includes excessive electrons and roll-off occurs.In that case, a carrier recombination region is narrowed, whichadversely affect the lifetime. However, by designing the LUMO levels ofthe host material, the material for the first electron-transport layer114-1, and the material for the second electron-transport layer 114-2 asdescribed above, the carrier balance can be maintained in the highluminance region and thus the emission efficiency can be increased evenwhen the hole-injection layer including an organic acceptor is used.Moreover, the carrier recombination region can be kept wide, and thusthe lifetime can be prolonged. An organic acceptor has an advantage ofexcellent sublimation property and a disadvantage of low hole-injectioncapability, but the problem can be solved by one embodiment of thepresent invention.

Furthermore, not only the lifetime but also the emission efficiency canbe improved depending on the structure. For example, in the case where acomposite material of a material having a hole-transport property and amaterial having an acceptor property (e.g., a transition metal oxidehaving an acceptor property, particularly an oxide of any of metalsbelonging to Group 4 to Group 8 in the periodic table) is used for thehole-injection layer 111, a light-emitting element showing excellentefficiency in not only the high luminance region but also in almost allthe region can be obtained.

Next, examples of specific structures and materials of theabove-described light-emitting element are described. As describedabove, the light-emitting element of one embodiment of the presentinvention includes the EL layer 103 that consists of a plurality oflayers between the anode 101 and the cathode 102; the EL layer 103includes at least the light-emitting layer 113, the firstelectron-transport layer 114-1, and the second electron-transport layer114-2. The light-emitting layer 113, the first electron-transport layer114-1, and the second electron-transport layer 114-2 are provided incontact with each other and in this order.

There is no particular limitation on layers other than thelight-emitting layer 113, the first electron-transport layer 114-1, andthe second electron-transport layer 114-2 included in the EL layer 103,and various layers such as a hole-injection layer, a hole-transportlayer, an electron-injection layer, a carrier-blocking layer, anexciton-blocking layer, and a charge-generation layer can be employed.

The anode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specific examples include indium oxide-tin oxide (ITO: indiumtin oxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, and indium oxide containing tungsten oxide andzinc oxide (IWZO). Such conductive metal oxide films are usually formedby a sputtering method, but may be formed by application of a sol-gelmethod or the like. In an example of the formation method, indiumoxide-zinc oxide is deposited by a sputtering method using a targetobtained by adding 1 wt % to 20 wt % of zinc oxide to indium oxide.Furthermore, a film of indium oxide containing tungsten oxide and zincoxide (IWZO) can be formed by a sputtering method using a target inwhich tungsten oxide and zinc oxide are added to indium oxide at 0.5 wt% to 5 wt % and 0.1 wt % to 1 wt %, respectively. Alternatively, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd),nitride of a metal material (e.g., titanium nitride), or the like can beused. Graphene can also be used. Note that when a composite materialdescribed later is used for a layer which is in contact with the anode101 in the EL layer 103, an electrode material can be selectedregardless of its work function.

In this embodiment, two types of stacked structures of the EL layer 103will be described. A structure illustrated in FIG. 1A includes thehole-injection layer 111, a hole-transport layer 112, the light-emittinglayer 113, the first electron-transport layer 114-1, the secondelectron-transport layer 114-2, and an electron-injection layer 115. Astructure illustrated in FIG. 1B includes the hole-injection layer 111,the hole-transport layer 112, the light-emitting layer 113, the firstelectron-transport layer 114-1, the second electron-transport layer114-2, and a charge-generation layer 116. Materials for forming thelayers are specifically shown below.

The hole-injection layer 111 is a layer containing a substance having ahole-injection property. For example, a transition metal oxide, inparticular, an oxide of a metal belonging to Group 4 to Group 8 in theperiodic table (e.g., a molybdenum oxide, a vanadium oxide, a rutheniumoxide, a rhenium oxide, a tungsten oxide, or a manganese oxide) can beused. Alternatively, a complex of a transition metal or a complex of anoxide of a metal belonging to Group 4 to Group 8 in the periodic tablecan be used; for example, a molybdenum complex such as molybdenumtris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (abbreviation:Mo(tfd)₃) can be used. The transition metal oxide (in particular, theoxide of a metal belonging to Group 4 to Group 8 in the periodic table)or the complex of a transition metal (in particular, the complex of anoxide of a metal belonging to Group 4 to Group 8 in the periodic table)acts as an acceptor. The acceptor can extract an electron from ahole-transport layer (or a hole-transport material) adjacent to thehole-injection layer 111 by at least application of an electric field.Further alternatively, a compound including an electron-withdrawinggroup (a halogen group or a cyano group) such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), 3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane, chloranil,or 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene(abbreviation: HAT-CN) can be used. The compound including anelectron-withdrawing group (a halogen group or a cyano group) acts as anorganic acceptor. The organic acceptor can extract an electron from ahole-transport layer (or a hole-transport material) adjacent to thehole-injection layer 111 by at least application of an electric field.Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N-diphenyl-(1,1-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or the like.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the anode 101. Examples of the acceptormaterial include a compound including an electron-withdrawing group (ahalogen group or a cyano group) such as F₄-TCNQ, chloranil, or HAT-CN, atransition metal oxide (in particular, an oxide of a metal belonging toGroup 4 to Group 8 in the periodic table). A transition metal oxide (inparticular, an oxide of a metal belonging to Group 4 to Group 8 in theperiodic table) is preferred because these oxides show an acceptorproperty with respect to a substance having a hole-transport propertywhose HOMO level is lower (deeper) than −5.4 eV (these oxides canextract an electron by at least application of an electric field).

As the compound including an electron-withdrawing group (a halogen groupor a cyano group), a compound in which an electron-withdrawing group isbonded to a condensed aromatic ring including a plurality of heteroatomssuch as HAT-CN is particularly preferred because of its thermalstability.

As the transition metal oxide or the oxide of a metal belonging to Group4 to Group 8 in the periodic table, vanadium oxide, niobium oxide,tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide are preferable because of their highacceptor properties. In particular, molybdenum oxide is more preferablebecause of its stability in the atmosphere, low hygroscopic property,and easiness of handling.

As the substance having a hole-transport property which is used for thecomposite material, any of a variety of organic compounds such asaromatic amine compounds, carbazole derivatives, aromatic hydrocarbons,and high molecular compounds (e.g., oligomers, dendrimers, or polymers)can be used. Note that the organic compound used for the compositematerial is preferably an organic compound having a high hole-transportproperty. Specifically, a substance having a hole mobility of 10⁻⁶cm²/Vs or more is preferably used. Examples of organic compounds thatcan be used as the substance having a hole-transport property in thecomposite material are specifically given below.

Examples of the aromatic amine compounds includeN,N-di(p-tolyl)-N,N-diphenyl-p-phenylenediamine (abbreviation: DTDPPA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), and1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B).

Examples of the carbazole derivative include3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Alternatively, 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, or the likecan be used.

Examples of the aromatic hydrocarbon include2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA);2-tert-butyl-9,10-di(1-naphthyl)anthracene;9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA);2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA);9,10-di(2-naphthyl)anthracene (abbreviation: DNA);9,10-diphenylanthracene (abbreviation: DPAnth); 2-tert-butylanthracene(abbreviation: t-BuAnth); 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA);2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene;9,10-bis[2-(1-naphthyl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene;2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene; 9,9′-bianthryl;10,10′-diphenyl-9,9′-bianthryl;10,10′-bis(2-phenylphenyl)-9,9′-bianthryl; 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; anthracene;tetracene; rubrene; perylene; and 2,5,8,11-tetra(tert-butyl)perylene.Besides, pentacene, coronene, or the like can be used. The aromatichydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or higher and having14 to 42 carbon atoms is particularly preferable. The aromatichydrocarbon may have a vinyl skeleton. Examples of an aromatichydrocarbon having a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi) and9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA).

A high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine] (abbreviation:poly-TPD) can also be used.

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer 112 contains a substance having ahole-transport property. Examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP).The substances mentioned here have a high hole-transport property andare mainly ones that have a hole mobility of 10⁻⁶ cm²/Vs or more. Anorganic compound given as an example of the substance having ahole-transport property in the composite material described above canalso be used for the hole-transport layer 112. A high molecular compoundsuch as poly(N-vinylcarbazole) (abbreviation: PVK) andpoly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also be used. Notethat the layer that contains the substance having a hole-transportproperty is not limited to a single layer, and may be a stack of two ormore layers including any of the above substances.

The light-emitting layer 113 may be a layer that contains a fluorescentsubstance and emits fluorescence, a layer that contains a phosphorescentsubstance and emits phosphorescence, or a layer that contains asubstance emitting thermally activated delayed fluorescence (TADF) andemits TADF. Furthermore, the light-emitting layer 113 may be a singlelayer or include a plurality of layers containing differentlight-emitting substances. Note that in one embodiment of the presentinvention, a layer that emits fluorescence, specifically, a layer thatemits blue fluorescence is preferred.

Examples of a material that can be used as a fluorescent substance inthe light-emitting layer 113 are as follows. Fluorescent substancesother than those given below can also be used.

The examples include5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N′-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA), N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-tiphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBCI), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-(2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD),7,14-diphenyl-N,N,N′,N-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluorantbene-3,10-diamine (abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM), and2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[q]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM). Condensed aromatic diamine compounds typifiedby pyrenediamine compounds such as 1,6FLPAPm and 1,6mMemFLPAPrn areparticularly preferable because of their high hole-trapping property,high emission efficiency, and high reliability.

Examples of materials that can be used as the phosphorescent substancein the light-emitting layer 113 are as follows.

The examples include organometallic iridium complexes having 4H-triazoleskeletons, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: [Ir(Mptz)₃]), andtris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: [Ir(iPrptz-3b)₃]); organometallic iridium complexeshaving 1H-triazole skeletons, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: [Ir(Mptz1-mp)₃]) andtris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: [Ir(Prptz1-Me)₃]); organometallic iridium complexeshaving imidazole skeletons, such asfac-tris[l-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: [Ir(iPrpmi)₃]) andtris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: [Ir(dmpimpt-Me)₃]); and organometallic iridium complexesin which aphenylpyridine derivative having an electron-withdrawing groupis a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(II) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C²}iridium(III)picolinate (abbreviation: [Ir(CF₃ppy)₂(pic)]), andbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²]iridium(III) acetylacetonate(abbreviation: FIr(acac)). These are compounds emitting bluephosphorescence and have an emission peak at 440 nm to 520 nm.

Other examples include organometallic iridium complexes havingpyrimidine skeletons, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:[Ir(mppm)₃]), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₃]),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(I)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(II)(abbreviation: [Ir(nbppm)₂(acac)]),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(II)(abbreviation: [Ir(mpmppm)₂(acac)]), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]); organometallic iridium complexeshaving pyrazine skeletons, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(acac)]) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-iPr)₂(acac)]); organometallic iridium complexeshaving pyridine skeletons, such astris(2-phenylpyridinato-N,C²′)iridium(III) (abbreviation: [Ir(ppy)₃]),bis(2-phenylpyridinato-N,C²′)iridium(III) acetylacetonate (abbreviation:[Ir(ppy)₂(acac)]), bis(benzo[h]quinolinato)iridium(III) acetylacetonate(abbreviation: [Ir(bzq)₂(acac)]), tris(benzo[h]quinolinato)iridium(II)(abbreviation: [Ir(bzq)₃]), tris(2-phenylquinolinato-N,C²′)iridium(III)(abbreviation: [Ir(pq)₃]), and bis(2-phenylquinolinato-N,C²′)iridium(II)acetylacetonate (abbreviation: [Ir(pq)₂(acac)]); and rare earth metalcomplexes such as tris(acetylacetonato)(monophenanthroline)terbium(III)(abbreviation: [Tb(acac)₃(Phen)]). These are mainly compounds emittinggreen phosphorescence and have an emission peak at 500 nm to 600 nm.Note that organometallic iridium complexes having pyrimidine skeletonshave distinctively high reliability and emission efficiency and thus areespecially preferable.

Other examples include organometallic iridium complexes havingpyrimidine skeletons, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(II)(abbreviation: [Ir(5mdppm)₂(dibm)]),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: [Ir(5mdppm)₂(dpm)]),bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: [Ir(d1npm)₂(dpm)]); organometallic iridium complexeshaving pyrazine skeletons, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]),bis(2,3,5-triphenylpyrazinatoxdipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]), and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(II)(abbreviation: [Ir(Fdpq)₂(acac)]); organometallic iridium complexeshaving pyridine skeletons, such astris(1-phenylisoquinolinato-N,C²′)iridium(III) (abbreviation:[Ir(piq)₃]) and bis(1-phenylisoquinolinato-N,C²′)iridium(III)acetylacetonate (abbreviation: [Ir(piq)₂(acac)]); platinum complexessuch as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and rare earth metal complexes such astris(1,3-diphenyl-1,3-propanedionatoXmonophenanthroline)europium(III)(abbreviation: [Eu(DBM)₃(Phen)]) andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: [Eu(TTA)₃(Phen)]). These are compounds emitting redphosphorescence and have an emission peak at 600 nm to 700 nm.Furthermore, organometallic iridium complexes having pyrazine skeletonscan provide red light emission with favorable chromaticity.

As well as the above phosphorescent compounds, known phosphorescentmaterials may be selected and used.

Examples of the TADF material include a fullerene, a derivative thereof,an acridine derivative such as proflavine, eosin, or the like, and ametal-containing porphyrin such as a porphyrin containing magnesium(Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), orpalladium (Pd). Examples of the metal-containing porphyrin include aprotoporphyrin-tin fluoride complex (SnF₂(Proto IX)), amesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂(OEP)), which areshown in the following structural formulae.

Alternatively, a heterocyclic compound having a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring, suchas2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(PIC-TRZ) shown in the following structural formula, can be used. Theheterocyclic compound is preferably used because of the π-electron richheteroaromatic ring and the π-electron deficient heteroaromatic ring,for which the electron-transport property and the hole-transportproperty are high. Note that a substance in which the π-electron richheteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because the donorproperty of the π-electron rich heteroaromatic ring and the acceptorproperty of the π-electron deficient heteroaromatic ring are bothincreased and the energy difference between the S₁ level and the T₁level becomes small.

A variety of carrier-transport materials can be used as the hostmaterial of the light-emitting layer. As the carrier-transport material,any of substances having a hole-transport property and substances havingan electron-transport property listed below and the like can be used. Itis needless to say that it is possible to use a material having ahole-transport property, a material having an electron-transportproperty, or a bipolar material other than the substances listed below.

The following are examples of materials having a hole-transportproperty: compounds having aromatic amine skeletons, such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBAIBP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), andN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); compounds having carbazole skeletons, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), and3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); compounds havingthiophene skeletons, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-m), and4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and compounds having furan skeletons, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II)and 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, the compoundshaving aromatic amine skeletons and the compounds having carbazoleskeletons are preferred because these compounds are highly reliable andhave a high hole-transport property and contribute to a reduction indrive voltage.

The following are examples of materials having an electron-transportproperty: metal complexes such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds having polyazole skeletons, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(I-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); heterocyclic compounds having diazineskeletons, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), and4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and heterocyclic compounds having pyridine skeletons,such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) and 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB). Among the above materials, the heterocyclic compounds havingdiazine skeletons and the heterocyclic compounds having pyridineskeletons are highly reliable and preferred. In particular, theheterocyclic compounds having diazine (pyrimidine or pyrazine) skeletonshave a high electron-transport property and contribute to a decrease indrive voltage.

Note that the host material may be a mixture of a plurality of kinds ofsubstances, and in the case of using a mixed host material, it ispreferable to mix a material having an electron-transport property witha material having a hole-transport property. By mixing the materialhaving an electron-transport property with the material having ahole-transport property, the transport property of the light-emittinglayer 113 can be easily adjusted and a recombination region can beeasily controlled. The ratio of the content of the material having ahole-transport property to the content of the material having anelectron-transport property may be 1:9 to 9:1.

An exciplex may be formed by these mixed materials. When these mixedmaterials are selected so as to form an exciplex that exhibits lightemission whose wavelength overlaps with the wavelength of alowest-energy-side absorption band of the light-emitting material,energy can be transferred smoothly and light emission can be obtainedefficiently. In addition, such a combination is preferable in thatdriving voltage can be reduced.

The electron-transport layer 114 contains a substance having anelectron-transport property. As the substance having anelectron-transport property, it is possible to use any of theabove-listed substances having an electron-transport property that canbe used as a host material.

An alkali metal, an alkaline earth metal, or a compound thereof such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂) may be provided between the electron-transport layer 114 and thecathode 102 as part of the cathode 102. For example, a layer that isformed using a substance having an electron-transport property and thatcontains an alkali metal, an alkaline earth metal, a compound thereof,or an electride can be used. Examples of the electride include asubstance in which electrons are added at high concentration to calciumoxide-aluminum oxide.

Here, examples of particularly preferable structures for thelight-emitting element of one embodiment of the present invention aredescribed.

In the light-emitting element of one embodiment of the presentinvention, the host material is preferably a substance including acondensed aromatic ring skeleton including 3 to 6 rings. Examples of thesubstance including a condensed aromatic ring skeleton including 3 to 6rings are substances including an anthracene skeleton such as CzPA,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA),9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), 4-[3-(9,10-diphenyl-2-anthryl)phenyl]dibenzofuran (abbreviation:2mDBFPPA-II), t-BuDNA, and9-(2-naphthyl)-10-[4-(1-naphthyl)phenyl]anthracene (abbreviation: BH-1);substances including a tetracene skeleton such as 5,12-diphenyltetracene(abbreviation: DPT), rubrene, and 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (abbreviation: TBRb);substances including a pyrene skeleton such as1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3),9,9-bis[4-(1-pyrenyl)phenyl]-9H-fluoren (abbreviation: BPPF), and2,7-bis(1-pyrenyl)-spiro-9,9′-bifluorene (abbreviation: Spyro-pye); asubstance including a perylene skeleton such as2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP); a substanceincluding a fluoranthene skeleton; and a substance including adibenzochrysene skeleton. Among these substances, substances includingan anthracene skeleton are particularly preferred as described above.

In the light-emitting element of one embodiment of the presentinvention, the electron-transport layer 114 preferably has a two-layerstructure of the first electron-transport layer 114-1 and the secondelectron-transport layer 114-2. The material for the firstelectron-transport layer 114-1 and the material for the secondelectron-transport layer 114-2 preferably include differentheteroaromatic ring skeletons, in which case the LUMO level of thematerial for the first electron-transport layer 114-1 can be lower(deeper) than the LUMO level of each of the host material and thematerial for the second electron-transport layer 114-2.

Note that the substance including a heteroaromatic ring skeleton for thefirst electron-transport layer 114-1 and that for the secondelectron-transport layer 114-2 are preferably substances including asix-membered nitrogen-containing heteroaromatic ring skeleton. Asubstance including a six-membered nitrogen-containing heteroaromaticskeleton has higher reliability as an electron acceptor than afive-membered nitrogen-containing heterocyclic skeleton (e.g., pyrrole,indole, carbazole, imidazole, benzimidazole, triazole, orbenzotriazole), leading to a light-emitting element having highreliability. A substance including a six-membered nitrogen-containingheteroaromatic skeleton is particularly suitable for the material forthe first electron-transport layer 114-1 because the substance includinga six-membered nitrogen-containing heteroaromatic skeleton tends to havea deeper LUMO level than a substance including a five-memberednitrogen-containing heterocyclic skeleton.

Therefore, the substance including a heteroaromatic ring skeleton forthe first electron-transport layer 114-1 preferably includes a triazineskeleton or a diazine skeleton (in particular, a pyrazine skeleton or apyrimidine skeleton). In particular, the substance including aheteroaromatic ring skeleton preferably includes a condensedheteroaromatic ring skeleton. Favorable examples the condensedheteroaromatic ring skeleton including a diazine skeleton include ahighly reliable benzoquinazoline skeleton or a dibenzoquinoxalineskeleton. In particular, a dibenzoquinoxaline skeleton is preferredbecause its LUMO level is easily deep. The light-emitting element of oneembodiment of the present invention with the above-described structurecan have a long lifetime, which shows less degradation due to anincrease in driving time.

The substance including a heteroaromatic ring skeleton for the secondelectron-transport layer 114-2 is preferably a substance including apyridine skeleton or a bipyridine skeleton in the case where the secondelectron-transport layer 114-2 is in contact with the cathode. In thecase where the substance including a heteroaromatic ring skeleton forthe first electron-transport layer 114-1 includes a triazine skeleton ora diazine skeleton (in particular, a pyrazine skeleton or a pyrimidineskeleton), a combination of the pyridine skeleton or the bipyridineskeleton with the triazine skeleton or the diazine skeleton is preferredbecause the LUMO level of the pyridine skeleton or the bipyridineskeleton is higher than the LUMO level of the triazine skeleton or thediazine skeleton. The pyridine skeleton or the bipyridine skeleton mayform a condensed ring, for example, may form a phenanthroline skeleton.

Examples of the substances including a heteroaromatic ring skeleton forthe first electron-transport layer 114-1 and the secondelectron-transport layer 114-2 include substances including adibenzoquinoxaline skeleton such as 2mDBTPDBq-II, 2mDBTBPDBq-II,2-{3-[3-(2,8-diphenyldibenzothiophen-4-yl)phenyl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-{3-[3-(6-phenyldibenzothiophen-4-yl)phenyl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-IV),2-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: PCPDBq),2-[3-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzPDBq-I),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), and7-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 7mDBTBPDBq-II); substances including a benzoquinazolineskeleton such as2,2′-(pyridine-2,6-diyl)bis(4-phenylbenzo[h]quinazoline) (abbreviation:2,6(P-Bqn)2Py); substances including a pyrimidine skeleton such as4,6mDBTP2Pm-II, 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine(abbreviation: 4,6mCzP2Pm),4-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]benzofuro[3,2-d]pyrimidine(abbreviation: 4mDBTBPBfpm-II),4-{3-[3′-(9H-carbazol-9-yl)]biphenyl-3-yl}benzofuro[3,2-d]pyrimidine(abbreviation: 4mCzBPBfPm),4,6-bis[3,5-di(pyridin-3-yl)phenyl]-2-methylpyrimidine (abbreviation:B3PYMPM), and 2,2′-(pyridine-2,6-diyl)bis(4,6-diphenylpyrimidine)(abbreviation: 2,6(P2Pm)2Py); substances including a pyrazine skeletonsuch as pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile(abbreviation: PPDN), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation:2PYPR), and 2,3-diphenylpyrido[3,4-b]pyrazine (abbreviation: 3PYPR);substances including a triazine skeleton such as2,4,6-tris(2-pyridyl)-1,3,5-triazine (abbreviation: 2Py3Tzn),2,4,6-tris[3′-(pyridin-3-yl)biphenyl-3-yl]-1,3,5-triazine (abbreviation:TmPPPyTz), and3-[4-(9H-carbazol-9-yl)phenyl]-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole(abbreviation: CPCBPTz); substances including a phenanthroline skeletonsuch as bathocuproine (abbreviation: BCP), bathophenanthroline(abbreviation: Bphen),2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen), and 4,4′-di(1,10-phenanthrolin-2-yl)biphenyl (abbreviation:Phen2BP); substances including a bipyridine skeleton such as4,4′-bis[3-(9H-carbazol-9-yl)phenyl]-2,2′-bipyridine (abbreviation:4,4′mCzP2BPy), 4,4′-bis[3-(dibenzothiophen-4-yl)phenyl]-2,2′-bipyridine(abbreviation: 4,4′mDBTP2BPy-II), and4,4′-bis[3-(dibenzofuran-4-yl)phenyl]-2,2′-bipyridine (abbreviation:4,4′DBfP2BPy-II); and substances including a pyridine skeleton such astris[2,4,6-trimethyl-3-(3-pyridyl)phenyl]borane (abbreviation: 3TPYMB),1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB),3,3′,5,5′-tetra[(m-pyridyl)-phen-3-yl]biphenyl (abbreviation: BP4mPy),and 1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene (abbreviation: BmPyPhB).Among these substances and the above-mentioned substances including acondensed aromatic ring skeleton including 3 to 6 rings which can beused as the host material, materials may be selected such that the LUMOlevel of each of the host material and the material for the secondelectron-transport layer 114-2 is higher (shallower) than the LUMO levelof the material for the first electron-transport layer 114-1.

Among the light-emitting elements with favorable structures, thelight-emitting element using a composite material of a metal oxide and asubstance having a hole-transport property for the hole-injection layer111 can have particularly high emission efficiency (e.g., externalquantum efficiency or current efficiency). Furthermore, thelight-emitting element of one embodiment of the present invention inwhich an organic acceptor, specifically, HAT-CN is used for thehole-injection layer can suppress the roll-off of the efficiency in thehigh luminance region, and thus the light-emitting element can achievehigh luminance and high efficiency.

Instead of the electron-injection layer 115, the charge-generation layer116 may be provided (FIG. 1B). The charge-generation layer 116 refers toa layer capable of injecting holes into a layer in contact with thecathode side of the charge-generation layer 116 and electrons into alayer in contact with the anode side thereof when a potential isapplied. The charge-generation layer 116 includes at least a p-typelayer 117. The p-type layer 117 is preferably formed using any of thecomposite materials given above as examples of materials that can beused for the hole-injection layer 111. The p-type layer 117 may beformed by stacking a film containing the above-described acceptormaterial as a material included in the composite material and a filmcontaining a hole-transport material. When a potential is applied to thep-type layer 117, electrons are injected into the electron-transportlayer 114 and holes are injected into the cathode 102; thus, thelight-emitting element operates.

Note that the charge-generation layer 116 preferably includes either anelectron-relay layer 118 or an electron-injection buffer layer 119 orboth in addition to the p-type layer 117.

The electron-relay layer 118 contains at least the substance having anelectron-transport property and has a function of preventing aninteraction between the electron-injection buffer layer 119 and thep-type layer 117 and smoothly transferring electrons. The LUMO level ofthe substance having an electron-transport property contained in theelectron-relay layer 118 is preferably between the LUMO level of thesubstance having an acceptor property in the p-type layer 117 and theLUMO level of a substance contained in a layer of the electron-transportlayer 114 in contact with the charge-generation layer 116. As a specificvalue of the energy level, the LUMO level of the substance having anelectron-transport property in the electron-relay layer 118 ispreferably higher than or equal to −5.0 eV, more preferably higher thanor equal to −5.0 eV and lower than or equal to −3.0 eV. Note that as thesubstance having an electron-transport property in the electron-relaylayer 118, a phthalocyanine-based material or a metal complex having ametal-oxygen bond and an aromatic ligand is preferably used.

A substance having a high electron-injection property can be used forthe electron-injection buffer layer 119. For example, an alkali metal,an alkaline earth metal, a rare earth metal, or a compound thereof(e.g., an alkali metal compound (including an oxide such as lithiumoxide, a halide, and a carbonate such as lithium carbonate or cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, and a carbonate), or a rare earth metal compound (including anoxide, a halide, and a carbonate)) can be used.

In the case where the electron-injection buffer layer 119 contains thesubstance having an electron-transport property and a donor substance,an organic compound such as tetrathianaphthacene (abbreviation: TIN),nickelocene, or decamethylnickelocene can be used as the donorsubstance, as well as an alkali metal, an alkaline earth metal, a rareearth metal, a compound of the above metal (e.g., an alkali metalcompound (including an oxide such as lithium oxide, a halide, and acarbonate such as lithium carbonate or cesium carbonate), an alkalineearth metal compound (including an oxide, a halide, and a carbonate),and a rare earth metal compound (including an oxide, a halide, and acarbonate)). Note that as the substance having an electron-transportproperty, a material similar to the above-described material used forthe electron-transport layer 114 can be used.

For the cathode 102, any of metals, alloys, electrically conductivecompounds, and mixtures thereof which have a low work function(specifically, a work function of 3.8 eV or less) or the like can beused. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thecathode 102 and the electron-transport layer, for the cathode 102, anyof a variety of conductive materials such as Al, Ag, ITO, or indiumoxide-tin oxide containing silicon or silicon oxide can be usedregardless of the work function. Films of these conductive materials canbe formed by a dry method such as a vacuum evaporation method or asputtering method, an inkjet method, a spin coating method, or the like.In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.

Any of a variety of methods can be used to form the EL layer 103regardless of whether it is a dry process or a wet process. For example,a vacuum evaporation method, a gravure printing method, an offsetprinting method, a screen printing method, an inkjet method, a spincoating method, or the like may be used.

Different methods may be used to form the electrodes or the layersdescribed above.

The structure of the layers provided between the anode 101 and thecathode 102 is not limited to the above-described structure. Preferably,a light-emitting region where holes and electrons recombine ispositioned away from the anode 101 and the cathode 102 so that quenchingdue to the proximity of the light-emitting region and a metal used forelectrodes and carrier-injection layers can be prevented.

Furthermore, in order that transfer of energy from an exciton generatedin the light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer closer to the recombination region in thelight-emitting layer 113, are formed using a substance having a widerband gap than the light-emitting substance of the light-emitting layeror the emission center substance included in the light-emitting layer.

Next, an embodiment of a light-emitting element with a structure inwhich a plurality of light-emitting units are stacked (this type oflight-emitting element is also referred to as a stacked element or atandem element) is described with reference to FIG. 1C. In thislight-emitting element, a plurality of light-emitting units are providedbetween an anode and a cathode. One light-emitting unit has a structuresimilar to that of the EL layer 103, which is illustrated in FIG. 1A. Inother words, the light-emitting element illustrated in FIG. 1C is alight-emitting element including a plurality of light-emitting units;each of the light-emitting elements illustrated in FIGS. 1A and 1B is alight-emitting element including a single light-emitting unit.

In FIG. 1C, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge-generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 correspond,respectively, to the anode 101 and the cathode 102 illustrated in FIG.1A, and the materials given in the description for FIG. 1A can be used.Furthermore, the first light-emitting unit 511 and the secondlight-emitting unit 512 may have the same structure or differentstructures.

The charge-generation layer 513 has a function of injecting electronsinto one of the light-emitting units and injecting holes into the otherof the light-emitting units when a voltage is applied between the firstelectrode 501 and the second electrode 502. That is, in FIG. 1C, thecharge-generation layer 513 injects electrons into the firstlight-emitting unit 511 and holes into the second light-emitting unit512 when a voltage is applied so that the potential of the firstelectrode becomes higher than the potential of the second electrode.

The charge-generation layer 513 preferably has a structure similar tothe charge-generation layer 116 described with reference to FIG. 1B.Since the composite material of an organic compound and a metal oxide issuperior in carrier-injection property and carrier-transport property,low-voltage driving or low-current driving can be achieved. Note thatwhen a surface of a light-emitting unit on the anode side is in contactwith the charge-generation layer 513, the charge-generation layer 513can also serve as a hole-injection layer in the light-emitting unit anda hole-transport layer is not necessarily formed in the light-emittingunit.

In the case where the electron-injection buffer layer 119 is provided,the electron-injection buffer layer 119 serves as the electron-injectionlayer in the light-emitting unit on the anode side which is in contactwith the electron-injection buffer layer 119 and the light-emitting uniton the anode side does not necessarily further need anelectron-injection layer.

The light-emitting element having two light-emitting units is describedwith reference to FIG. 1C; however, the present invention can besimilarly applied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge-generation layer 513 between a pair ofelectrodes as in the light-emitting element according to thisembodiment, it is possible to provide a light-emitting element which canemit light with high luminance with the current density kept low and hasa long lifetime. Moreover, a light-emitting device having low powerconsumption, which can be driven at low voltage, can be manufactured.

Furthermore, when emission colors of light-emitting units are madedifferent, light emission of a desired color can be provided from thelight-emitting element as a whole. For example, in a light-emittingelement having two light-emitting units, the emission colors of thefirst light-emitting unit may be red and green and the emission color ofthe second light-emitting unit may be blue, so that the light-emittingelement can emit white light as the whole element.

The above-described structure can be combined with any of the structuresin this embodiment and the other embodiments.

Embodiment 2

In this embodiment, a light-emitting device including the light-emittingelement containing any of the organic compounds described in Embodiment1 is described.

In this embodiment, the light-emitting device manufactured using thelight-emitting element containing any of the organic compounds describedin Embodiment 1 is described with reference to FIGS. 2A and 2B. Notethat FIG. 2A is a top view of the light-emitting device and FIG. 2B is across-sectional view taken along the lines A-B and C-D in FIG. 2A. Thislight-emitting device includes a driver circuit portion (source linedriver circuit) 601, a pixel portion 602, and a driver circuit portion(gate line driver circuit) 603, which are to control light emission of alight-emitting element and illustrated with dotted lines. Referencenumeral 604 denotes a sealing substrate; 605, a sealing material; and607, a space surrounded by the sealing material 605.

Reference numeral 608 denotes a lead wiring for transmitting signals tobe input to the source line driver circuit 601 and the gate line drivercircuit 603 and receiving signals such as a video signal, a clocksignal, a start signal, and a reset signal from a flexible printedcircuit (FPC) 609 serving as an external input terminal. Although onlythe FPC is illustrated here, a printed wiring board (PWB) may beattached to the FPC. The light-emitting device in the presentspecification includes, in its category, not only the light-emittingdevice itself but also the light-emitting device provided with the FPCor the PWB.

Next, a cross-sectional structure will be described with reference toFIG. 2B. The driver circuit portions and the pixel portion are formedover an element substrate 610; FIG. 2B shows the source line drivercircuit 601, which is a driver circuit portion, and one pixel in thepixel portion 602.

The element substrate 610 may be a substrate containing glass, quartz,an organic resin, a metal, an alloy, or a semiconductor or a plasticsubstrate formed of fiber reinforced plastic (FRP), poly(vinyl fluoride)(PVF), polyester, or acrylic.

The structure of transistors used in pixels and driver circuits is notparticularly limited. For example, inverted staggered transistors may beused, or staggered transistors may be used. Furthermore, top-gatetransistors or bottom-gate transistors may be used. A semiconductormaterial used for the transistors is not particularly limited, and forexample, silicon, germanium, silicon carbide, gallium nitride, or thelike can be used. Alternatively, an oxide semiconductor containing atleast one of indium, gallium, and zinc, such as an In—Ga—Zn-based metaloxide, may be used.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistors, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. It is preferable that a semiconductorhaving crystallinity be used, in which case deterioration of thetransistor characteristics can be suppressed.

Here, an oxide semiconductor is preferably used for semiconductordevices such as the transistors provided in the pixels and drivercircuits and transistors used for touch sensors described later, and thelike. In particular, an oxide semiconductor having a wider band gap thansilicon is preferably used. When an oxide semiconductor having a widerband gap than silicon is used, off-state current of the transistors canbe reduced.

The oxide semiconductor preferably contains at least indium (In) or zinc(Zn). Further preferably, the oxide semiconductor contains an oxiderepresented by an In-M-Zn-based oxide (M represents a metal such as Al,Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf).

As a semiconductor layer, it is particularly preferable to use an oxidesemiconductor film including a plurality of crystal parts whose c-axesare aligned perpendicular to a surface on which the semiconductor layeris formed or the top surface of the semiconductor layer and in which theadjacent crystal parts have no grain boundary.

The use of such materials for the semiconductor layer makes it possibleto provide a highly reliable transistor in which a change in theelectrical characteristics is suppressed.

Charge accumulated in a capacitor through a transistor including theabove-described semiconductor layer can be held for a long time becauseof the low off-state current of the transistor. When such a transistoris used in a pixel, operation of a driver circuit can be stopped while agray scale of an image displayed in each display region is maintained.As a result, an electronic device with extremely low power consumptioncan be obtained.

For stable characteristics of the transistor, a base film is preferablyprovided. The base film can be formed with a single-layer structure or astacked-layer structure using an inorganic insulating film such as asilicon oxide film, a silicon nitride film, a silicon oxynitride film,or a silicon nitride oxide film. The base film can be formed by asputtering method, a chemical vapor deposition (CVD) method (e.g., aplasma CVD method, a thermal CVD method, or a metal organic CVD (MOCVD)method), an atomic layer deposition (ALD) method, a coating method, aprinting method, or the like. Note that the base film is not necessarilyprovided.

Note that an FET 623 is illustrated as a transistor formed in the drivercircuit portion 601. In addition, the driver circuit may be formed withany of a variety of circuits such as a CMOS circuit, a PMOS circuit, oran NMOS circuit. Although a driver integrated type in which the drivercircuit is formed over the substrate is illustrated in this embodiment,the driver circuit is not necessarily formed over the substrate, and thedriver circuit can be formed outside, not over the substrate.

The pixel portion 602 includes a plurality of pixels including aswitching FET 611, a current controlling FET 612, and a first electrode613 electrically connected to a drain of the current controlling FET612. One embodiment of the present invention is not limited to thestructure. The pixel portion 602 may include three or more FETs and acapacitor in combination.

Note that to cover an end portion of the first electrode 613, aninsulator 614 is formed, for which a positive photosensitive acrylicresin film is used here.

In order to improve coverage with an EL layer or the like which isformed later, the insulator 614 is formed to have a curved surface withcurvature at its upper or lower end portion. For example, in the casewhere positive photosensitive acrylic is used as a material of theinsulator 614, only the upper end portion of the insulator 614preferably has a curved surface with a curvature radius (0.2 μm to 3μm). As the insulator 614, either a negative photosensitive resin or apositive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, as a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance, favorable ohmic contact, and a function as an anode.

The EL layer 616 is formed by any of a variety of methods such as anevaporation method using an evaporation mask, an inkjet method, and aspin coating method. The EL layer 616 has the structure described inEmbodiment 1. As another material included in the EL layer 616, a lowmolecular compound or a high molecular compound (including an oligomeror a dendrimer) may be used.

As a material used for the second electrode 617, which is formed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, and Ca, or an alloy or a compoundthereof, such as MgAg, MgIn, and AlLi) is preferably used. In the casewhere light generated in the EL layer 616 is transmitted through thesecond electrode 617, a stack of a thin metal film and a transparentconductive film (e.g., ITO, indium oxide containing zinc oxide at 2 wt %to 20 wt %, indium tin oxide containing silicon, or zinc oxide (ZnO)) ispreferably used for the second electrode 617.

Note that the light-emitting element is formed with the first electrode613, the EL layer 616, and the second electrode 617. The light-emittingelement is the light-emitting element described in Embodiment 1. In thelight-emitting device of this embodiment, the pixel portion, whichincludes a plurality of light-emitting elements, may include both thelight-emitting element described in Embodiment 1 and a light-emittingelement having a different structure.

The sealing substrate 604 is attached to the element substrate 610 withthe sealing material 605, so that a light-emitting element 618 isprovided in the space 607 surrounded by the element substrate 610, thesealing substrate 604, and the sealing material 605. The space 607 maybe filled with a filler, or may be filled with an inert gas (such asnitrogen or argon), or the sealing material. It is preferable that thesealing substrate be provided with a recessed portion and a drying agentbe provided in the recessed portion, in which case deterioration due toinfluence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material not be permeable tomoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate formed offiber reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester,and acrylic can be used.

Although not illustrated in FIGS. 2A and 2B, a protective film may beprovided over the second electrode. As the protective film, an organicresin film or an inorganic insulating film may be formed. The protectivefilm may be formed so as to cover an exposed portion of the sealingmaterial 605. The protective film may be provided so as to coversurfaces and side surfaces of the pair of substrates and exposed sidesurfaces of a sealing layer, an insulating layer, and the like.

The protective film can be formed using a material through which animpurity such as water does not permeate easily. Thus, diffusion of animpurity such as water from the outside into the inside can beeffectively suppressed.

As a material of the protective film, an oxide, a nitride, a fluoride, asulfide, a ternary compound, a metal, a polymer, or the like can beused. For example, the material may contain aluminum oxide, hafniumoxide, hafnium silicate, lanthanum oxide, silicon oxide, strontiumtitanate, tantalum oxide, titanium oxide, zinc oxide, niobium oxide,zirconium oxide, tin oxide, yttrium oxide, cerium oxide, scandium oxide,erbium oxide, vanadium oxide, indium oxide, aluminum nitride, hafniumnitride, silicon nitride, tantalum nitride, titanium nitride, niobiumnitride, molybdenum nitride, zirconium nitride, gallium nitride, anitride containing titanium and aluminum, an oxide containing titaniumand aluminum, an oxide containing aluminum and zinc, a sulfidecontaining manganese and zinc, a sulfide containing cerium andstrontium, an oxide containing erbium and aluminum, an oxide containingyttrium and zirconium, or the like.

The protective film is preferably formed using a deposition method withfavorable step coverage. One such method is an atomic layer deposition(ALD) method. A material that can be deposited by an ALD method ispreferably used for the protective film. A dense protective film havingreduced defects such as cracks or pinholes or a uniform thickness can beformed by an ALD method. Furthermore, damage caused to a process memberin forming the protective film can be reduced.

By an ALD method, a uniform protective film with few defects can beformed even on, for example, a surface with a complex uneven shape orupper, side, and lower surfaces of a touch panel.

As described above, the light-emitting device manufactured using thelight-emitting element described in Embodiment 1 can be obtained.

The light-emitting device in this embodiment is manufactured using thelight-emitting element described in Embodiment 1 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 has a long lifetime, thelight-emitting device can have high reliability. Since thelight-emitting device using the light-emitting element described inEmbodiment 1 has high emission efficiency, the light-emitting device canachieve low power consumption.

FIGS. 3A and 3B each illustrate an example of a light-emitting device inwhich full color display is achieved by formation of a light-emittingelement exhibiting white light emission and with the use of coloringlayers (color filters) and the like. In FIG. 3A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 1024G, and 1024B of light-emitting elements, a partition 1025, anEL layer 1028, a second electrode 1029 of the light-emitting elements, asealing substrate 1031, a sealing material 1032, and the like areillustrated.

In FIG. 3A, coloring layers (a red coloring layer 1034R, a greencoloring layer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black matrix 1035 may be additionallyprovided. The transparent base material 1033 provided with the coloringlayers and the black matrix is aligned and fixed to the substrate 1001.Note that the coloring layers and the black matrix 1035 are covered withan overcoat layer 1036. In FIG. 3A, light emitted from part of thelight-emitting layer does not pass through the coloring layers, whilelight emitted from the other part of the light-emitting layer passesthrough the coloring layers. Since light which does not pass through thecoloring layers is white and light which passes through any one of thecoloring layers is red, green, or blue, an image can be displayed usingpixels of the four colors.

FIG. 3B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the gate insulating film 1003and the first interlayer insulating film 1020. As in the structure, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where FETs are formed (a bottom emission structure), but may be alight-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 4is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offorming a connection electrode which connects the FET and the anode ofthe light-emitting element is performed in a manner similar to that ofthe light-emitting device having a bottom emission structure. Then, athird interlayer insulating film 1037 is formed to cover an electrode1022. This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, and can alternativelybe formed using any of other known materials.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each serve as an anode here, but may serve as acathode. Furthermore, in the case of a light-emitting device having atop emission structure as illustrated in FIG. 4, the first electrodesare preferably reflective electrodes. The EL layer 1028 is formed tohave a structure similar to the structure of the EL layer 103, which isdescribed in Embodiment 1, with which white light emission can beobtained.

In the case of a top emission structure as illustrated in FIG. 4,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black matrix 1035 which ispositioned between pixels. The coloring layers (the red coloring layer1034R, the green coloring layer 1034G, and the blue coloring layer1034B) and the black matrix may be covered with the overcoat layer. Notethat a light-transmitting substrate is used as the sealing substrate1031. Although an example in which full color display is performed usingfour colors of red, green, blue, and white is shown here, there is noparticular limitation and full color display using four colors of red,yellow, green, and blue or three colors of red, green, and blue may beperformed.

In the light-emitting device having a top emission structure, amicrocavity structure can be favorably employed. A light-emittingelement with a microcavity structure is formed with the use of areflective electrode as the first electrode and a semi-transmissive andsemi-reflective electrode as the second electrode. The light-emittingelement with a microcavity structure includes at least an EL layerbetween the reflective electrode and the semi-transmissive andsemi-reflective electrode, which includes at least a light-emittinglayer serving as a light-emitting region.

Note that the reflective electrode has a visible light reflectivity of40% to 100%, preferably 70% to 100%, and a resistivity of 1×10⁻² Ωcm orlower. In addition, the semi-transmissive and semi-reflective electrodehas a visible light reflectivity of 20% to 80%, preferably 40% to 70%,and a resistivity of 1×10⁻² Ωcm or lower.

Light emitted from the light-emitting layer included in the EL layer isreflected and resonated by the reflective electrode and thesemi-transmissive and semi-reflective electrode.

In the light-emitting element, by changing thicknesses of thetransparent conductive film, the composite material, thecarrier-transport material, and the like, the optical path lengthbetween the reflective electrode and the semi-transmissive andsemi-reflective electrode can be changed. Thus, light with a wavelengththat is resonated between the reflective electrode and thesemi-transmissive and semi-reflective electrode can be intensified whilelight with a wavelength that is not resonated therebetween can beattenuated.

Note that light that is reflected back by the reflective electrode(first reflected light) considerably interferes with light that directlyenters the semi-transmissive and semi-reflective electrode from thelight-emitting layer (first incident light). For this reason, theoptical path length between the reflective electrode and thelight-emitting layer is preferably adjusted to (2n−1)λ/4 (n is a naturalnumber of 1 or larger and λ is a wavelength of color to be amplified).By adjusting the optical path length, the phases of the first reflectedlight and the first incident light can be aligned with each other andthe light emitted from the light-emitting layer can be furtheramplified.

Note that in the above structure, the EL layer may include a pluralityof light-emitting layers or may include a single light-emitting layer.The tandem light-emitting element described above may be combined with aplurality of EL layers; for example, a light-emitting element may have astructure in which a plurality of EL layers are provided, acharge-generation layer is provided between the EL layers, and each ELlayer includes a plurality of light-emitting layers or a singlelight-emitting layer.

With the microcavity structure, emission intensity with a specificwavelength in the front direction can be increased, whereby powerconsumption can be reduced. Note that in the case of a light-emittingdevice which displays images with subpixels of four colors, red, yellow,green, and blue, the light-emitting device can have favorablecharacteristics because the luminance can be increased owing to yellowlight emission and each subpixel can employ a microcavity structuresuitable for wavelengths of the corresponding color.

The light-emitting device in this embodiment is manufactured using thelight-emitting element described in Embodiment 1 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 has a long lifetime, thelight-emitting device can have high reliability. Since thelight-emitting device using the light-emitting element described inEmbodiment 1 has high emission efficiency, the light-emitting device canachieve low power consumption.

An active matrix light-emitting device is described above, whereas apassive matrix light-emitting device is described below. FIGS. 5A and 5Billustrate a passive matrix light-emitting device manufactured using thepresent invention. Note that FIG. 5A is a perspective view of thelight-emitting device, and FIG. 5B is a cross-sectional view taken alongthe line X-Y in FIG. 5A. In FIGS. 5A and 5B, over a substrate 951, an ELlayer 955 is provided between an electrode 952 and an electrode 956. Anend portion of the electrode 952 is covered with an insulating layer953. A partition layer 954 is provided over the insulating layer 953.The sidewalls of the partition layer 954 are aslope such that thedistance between both sidewalls is gradually narrowed toward the surfaceof the substrate. In other words, a cross section taken along thedirection of the short side of the partition layer 954 is trapezoidal,and the lower side (a side of the trapezoid which is parallel to thesurface of the insulating layer 953 and is in contact with theinsulating layer 953) is shorter than the upper side (a side of thetrapezoid which is parallel to the surface of the insulating layer 953and is not in contact with the insulating layer 953). The partitionlayer 954 thus provided can prevent defects in the light-emittingelement due to static electricity or others. The passive-matrixlight-emitting device also includes the light-emitting element describedin Embodiment 1; thus, the light-emitting device can have highreliability or low power consumption.

Since many minute light-emitting elements arranged in a matrix in thelight-emitting device described above can each be controlled, thelight-emitting device can be suitably used as a display device fordisplaying images.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In this embodiment, an example in which the light-emitting elementdescribed in Embodiment 1 is used for a lighting device will bedescribed with reference to FIGS. 6A and 6B. FIG. 6B is a top view ofthe lighting device, and FIG. 6A is a cross-sectional view taken alongthe line e-f in FIG. 6B.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to theanode 101 in Embodiment 1. When light is extracted through the firstelectrode 401 side, the first electrode 401 is formed using a materialhaving a light-transmitting property.

A pad 412 for applying voltage to a second electrode 404 is providedover the substrate 400.

An EL layer 403 is formed over the first electrode 401. The structure ofthe EL layer 403 corresponds to, for example, the structure of the ELlayer 103 in Embodiment 1, or the structure in which the light-emittingunits 511 and 512 and the charge-generation layer 513 are combined.Refer to the descriptions for the structure.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the cathode 102 in Embodiment 1. The secondelectrode 404 is formed using a material having high reflectance whenlight is extracted through the first electrode 401 side. The secondelectrode 404 is connected to the pad 412, whereby voltage is applied.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404. Since the light-emittingelement is a light-emitting element with high emission efficiency, thelighting device in this embodiment can be a lighting device having lowpower consumption.

The substrate 400 provided with the light-emitting element having theabove structure is fixed to a sealing substrate 407 with sealingmaterials 405 and 406 and sealing is performed, whereby the lightingdevice is completed. It is possible to use only either the sealingmaterial 405 or the sealing material 406. The inner sealing material 406(not shown in FIG. 6B) can be mixed with a desiccant which enablesmoisture to be adsorbed, increasing reliability.

When parts of the pad 412 and the first electrode 401 are extended tothe outside of the sealing materials 405 and 406, the extended parts canserve as external input terminals. An IC chip 420 mounted with aconverter or the like may be provided over the external input terminals.

The lighting device described in this embodiment includes as an ELelement the light-emitting element described in Embodiment 1; thus, thelight-emitting device can have high reliability. The light-emittingdevice can also have high heat resistance.

Embodiment 4

In this embodiment, examples of electronic devices each including thelight-emitting element described in Embodiment 1 are described. Thelight-emitting element described in Embodiment 1 has a long lifetime andhigh reliability. As a result, the electronic devices described in thisembodiment can each include a light-emitting portion having highreliability.

Examples of the electronic devices to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic devices are given below.

FIG. 7A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Here,the housing 7101 is supported by a stand 7105. Images can be displayedon the display portion 7103, and in the display portion 7103, thelight-emitting elements described in Embodiment 1 are arranged in amatrix.

Operation of the television device can be performed with an operationswitch of the housing 7101 or a separate remote controller 7110. Withoperation keys 7109 of the remote controller 7110, channels and volumecan be controlled and images displayed on the display portion 7103 canbe controlled. The remote controller 7110 may be provided with a displayportion 7107 for displaying data output from the remote controller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 7B 1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by arranging light-emitting elementsdescribed in Embodiment 1 in a matrix in the display portion 7203. Thecomputer illustrated in FIG. 7B1 may have a structure illustrated inFIG. 7B2. The computer illustrated in FIG. 7B2 is provided with a seconddisplay portion 7210 instead of the keyboard 7204 and the pointingdevice 7206. The second display portion 7210 has a touch screen, andinput can be performed by operation of images, which are displayed onthe second display portion 7210, with a finger or a dedicated pen. Thesecond display portion 7210 can also display images other than thedisplay for input. The display portion 7203 may also have a touchscreen. Connecting the two screens with a hinge can prevent troubles;for example, the screens can be prevented from being cracked or brokenwhile the computer is being stored or carried.

FIG. 7C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be folded. Thehousing 7301 incorporates a display portion 7304 in which thelight-emitting elements described in Embodiment 1 are arranged in amatrix, and the housing 7302 incorporates a display portion 7305. Inaddition, the portable game machine illustrated in FIG. 7C includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring or sensingforce, displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), and a microphone 7312), and thelike. Needless to say, the structure of the portable game machine is notlimited to the above as long as the display portion in which thelight-emitting elements described in Embodiment 1 are arranged in amatrix is used as either the display portion 7304 or the display portion7305, or both, and the structure can include other accessories asappropriate. The portable game machine illustrated in FIG. 7C has afunction of reading out a program or data stored in a recoding medium todisplay it on the display portion, and a function of sharing informationwith another portable game machine by wireless communication. Note thatfunctions of the portable game machine illustrated in FIG. 7C are notlimited to them, and the portable game machine can have variousfunctions.

FIG. 7D illustrates an example of a portable terminal. The mobile phoneis provided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400has the display portion 7402 in which the light-emitting elementsdescribed in Embodiment 1 are arranged in a matrix.

When the display portion 7402 of the portable terminal illustrated inFIG. 7D is touched with a finger or the like, data can be input into theportable terminal. In this case, operations such as making a call andcreating an e-mail can be performed by touching the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating an e-mail, acharacter input mode is selected for the display portion 7402 so thatcharacters displayed on a screen can be input. In this case, it ispreferable to display a keyboard or number buttons on almost the entirescreen of the display portion 7402.

When a sensing device including a sensor such as a gyroscope or anacceleration sensor for detecting inclination is provided inside theportable terminal, display on the screen of the display portion 7402 canbe automatically changed in direction by determining the orientation ofthe portable terminal (whether the portable terminal is placedhorizontally or vertically).

The screen modes are switched by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401. Thescreen modes can be switched depending on the kind of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by thedisplay portion 7402 while in touch with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or a sensing light source which emits near-infrared light inthe display portion, an image of a finger vein, or a palm vein can betaken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

As described above, the application range of the light-emitting deviceincluding the light-emitting element described in Embodiment 1 is wideso that this light-emitting device can be applied to electronic devicesin a variety of fields. By using the light-emitting element described inEmbodiment 1, an electronic device having high reliability can beobtained.

FIG. 8 illustrates an example of a liquid crystal display device usingthe light-emitting element described in Embodiment 1 for a backlight.The liquid crystal display device illustrated in FIG. 8 includes ahousing 901, a liquid crystal layer 902, a backlight unit 903, and ahousing 904. The liquid crystal layer 902 is connected to a driver IC905. The light-emitting element described in Embodiment 1 is used forthe backlight unit 903, to which current is supplied through a terminal906.

The light-emitting element described in Embodiment 1 is used for thebacklight of the liquid crystal display device; thus, the backlight canhave reduced power consumption. In addition, the use of thelight-emitting element described in Embodiment 1 enables manufacture ofa planar-emission lighting device and further a larger-areaplanar-emission lighting device; therefore, the backlight can be alarger-area backlight, and the liquid crystal display device can also bea larger-area device. Furthermore, the light-emitting device includingthe light-emitting element described in Embodiment 1 can be thinner thana conventional one; accordingly, the display device can also be thinner.

FIG. 9 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 is used for a table lamp which is a lightingdevice. The table lamp illustrated in FIG. 9 includes a housing 2001 anda light source 2002, and the lighting device described in Embodiment 3may be used for the light source 2002.

FIG. 10 illustrates an example in which the light-emitting elementcontaining the organic compound described in Embodiment 1 is used for anindoor lighting device 3001. Since the light-emitting element containingthe organic compound described in Embodiment 1 has high heat resistance,the lighting device can have high heat resistance. Furthermore, sincethe light-emitting element containing the organic compound described inEmbodiment 1 can have a large area, the light-emitting element can beused for a large-area lighting device. Furthermore, since thelight-emitting element containing the organic compound described inEmbodiment 1 is thin, the light-emitting element can be used for alighting device having a reduced thickness.

The light-emitting element described in Embodiment 1 can also be usedfor an automobile windshield or an automobile dashboard. FIG. 11illustrates one mode in which the light-emitting element described inEmbodiment 1 is used for an automobile windshield and an automobiledashboard. Display regions 5000 to 5005 each include the light-emittingelement described in Embodiment 1.

The display region 5000 and the display region 5001 are display devicesprovided in the automobile windshield in which the light-emittingelements described in Embodiment 1 are incorporated. The light-emittingelements described in Embodiment 1 can be formed into what is called asee-through display device, through which the opposite side can be seen,by including a first electrode and a second electrode formed ofelectrodes having a light-transmitting property. Such see-throughdisplay devices can be provided even in the automobile windshield,without hindering the vision. Note that in the case where a drivingtransistor or the like is provided, a transistor having alight-transmitting property, such as an organic transistor using anorganic semiconductor material or a transistor using an oxidesemiconductor, is preferably used.

The display region 5002 is a display device provided in a pillar portionin which the light-emitting elements described in Embodiment 1 areincorporated. The display region 5002 can compensate for the viewhindered by the pillar portion by showing an image taken by an imagingunit provided in the car body. Similarly, the display region 5003provided in the dashboard can compensate for the view hindered by thecar body by showing an image taken by an imaging unit provided in theoutside of the car body, which leads to elimination of blind areas andenhancement of safety. Showing an image so as to compensate for the areawhich a driver cannot see makes it possible for the driver to confirmsafety easily and comfortably.

The display region 5004 and the display region 5005 can provide avariety of kinds of information such as navigation data, a speedmeter, atachometer, a mileage, a fuel level, a gearshift state, andair-condition setting. The content or layout of the display can befreely changed by a user as appropriate. Note that such information canalso be shown by the display regions 5000 to 5003. The display regions5000 to 5005 can also be used as lighting devices.

FIGS. 12A and 12B illustrate an example of a foldable tablet terminal.FIG. 12A illustrates the tablet terminal which is unfolded. The tabletterminal includes a housing 9630, a display portion 9631 a, a displayportion 9631 b, a display mode switch 9034, a power switch 9035, apower-saving mode switch 9036, a clasp 9033, and an operation switch.Note that in the tablet terminal, one or both of the display portion9631 a and the display portion 9631 b is/are formed using alight-emitting device which includes the light-emitting elementdescribed in Embodiment 1.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardcan be displayed on the entire region of the display portion 9631 a sothat the display portion 9631 a is used as a touchscreen, and thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. When a switching button 9639 forshowing/hiding a keyboard on the touchscreen is touched with a finger, astylus, or the like, the keyboard can be displayed on the displayportion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power-saving mode switch 9036 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal sensed by an optical sensorincorporated in the tablet terminal. Another sensing device including asensor such as a gyroscope or an acceleration sensor for sensinginclination may be incorporated in the tablet terminal, in addition tothe optical sensor.

Although FIG. 12A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, higherdefinition images may be displayed on one of the display portions 9631 aand 9631 b.

FIG. 12B illustrates the tablet terminal which is folded. The tabletterminal in this embodiment includes the housing 9630, a solar cell9633, a charge and discharge control circuit 9634, a battery 9635, and aDCDC converter 9636. In FIG. 12B, a structure including the battery 9635and the DCDC converter 9636 is illustrated as an example of the chargeand discharge control circuit 9634.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not in use. As a result, the display portion9631 a and the display portion 9631 b can be protected, therebyproviding a tablet terminal with high endurance and high reliability forlong-term use.

The tablet terminal illustrated in FIGS. 12A and 12B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touchscreen, the display portion, a video signalprocessing portion, or the like. Note that a structure in which thesolar cell 9633 is provided on one or both surfaces of the housing 9630is preferable because the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 12B will be described with reference to a blockdiagram of FIG. 12C. FIG. 12C illustrates the solar cell 9633, thebattery 9635, the DCDC converter 9636, a converter 9638, switches SW1 toSW3, and a display portion 9631. The battery 9635, the DCDC converter9636, the converter 9638, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 illustrated in FIG. 12B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 with the use of externallight. The voltage of the power generated by the solar cell is raised orlowered by the DCDC converter 9636 so as to be voltage for charging thebattery 9635. Then, when power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9638 soas to be voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module capable of performing charging bytransmitting and receiving power wirelessly (without contact), or any ofthe other charge means used in combination, and the power generationmeans is not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 12A to 12C as long as thedisplay portion 9631 is included.

FIGS. 13A to 13C illustrate a foldable portable information terminal9310. FIG. 13A illustrates the portable information terminal 9310 thatis opened. FIG. 13B illustrates the portable information terminal 9310that is being opened or being folded. FIG. 13C illustrates the portableinformation terminal 9310 that is folded. The portable informationterminal 9310 is highly portable when folded. When the portableinformation terminal 9310 is opened, a seamless large display region ishighly browsable.

A display panel 9311 is supported by three housings 9315 joined togetherby hinges 9313. The display panel 9311 may be a touch panel (aninput/output device) including a touch sensor (an input device). Bybending the display panel 9311 at a connection portion between twohousings 9315 with the use of the hinges 9313, the portable informationterminal 9310 can be reversibly changed in shape from an opened state toa folded state. The light-emitting device of one embodiment of thepresent invention can be used for the display panel 9311. A displayregion 9312 in the display panel 9311 includes a display region that ispositioned at a side surface of the portable information terminal 9310that is folded. On the display region 9312, information icons,frequently-used applications, file shortcuts to programs, and the likecan be displayed, and confirmation of information and start ofapplication can be smoothly performed.

Example 1

In this example, a light-emitting element 1 of one embodiment of thepresent invention, which corresponds to the light-emitting elementdescribed in Embodiment 1, and a comparative light-emitting element 1which has a different structure from the light-emitting element of oneembodiment of the present invention were compared, and the results aredescribed. A difference between the light-emitting element 1 and thecomparative light-emitting element 1 is only a material for the firstelectron-transport layer. Structural formulae of organic compounds usedin the light-emitting element 1 and the comparative light-emittingelement 1 are shown below.

(Method of Fabricating Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the anode101 was formed. The thickness thereof was 110 nm and the electrode areawas 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for an hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Next, the substrate provided with the anode 101 was fixed to a substrateholder provided in the vacuum evaporation device such that the side onwhich the anode 101 was formed faced downward. After the pressure waslowered to approximately 10⁻⁴ Pa,2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN) represented by the structural formula (i) was deposited to athickness of 10 nm over the anode 101 by an evaporation method usingresistance heating, whereby the hole-injection layer 111 was formed.

Next, over the hole-injection layer 111,N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula (ii)was deposited to a thickness of 10 nm to form the hole-transport layer112.

Furthermore, over the hole-transport layer 112, the light-emitting layer113 was formed by co-evaporation of7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by the above structural formula(iii) and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by the above structuralformula (iv) in a weight ratio of 1:0.05 (=cgDBCzPA: 1,6mMemFLPAPrn) toa thickness of 25 nm.

After that,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by the structural formula (v)was deposited to a thickness of 10 nm as the first electron-transportlayer 114-1 over the light-emitting layer 113, and thenbathophenanthroline (abbreviation: BPhen) represented by the structuralformula (vi) was deposited to a thickness of 15 nm as the secondelectron-transport layer 114-2.

After the formation of the first electron-transport layer 114-1 and thesecond electron-transport layer 114-2, lithium fluoride (LiF) wasdeposited by evaporation to a thickness of 1 nm and aluminum wasdeposited by evaporation to a thickness of 200 nm to form the cathode102. Thus, the light-emitting element 1 in this example was fabricated.

(Method for Fabricating Comparative Light-Emitting Element 1)

The comparative light-emitting element 1 was fabricated in the samemanner as the light-emitting element 1 except for changing 2mDBTBPDBq-IIin the first electron-transport layer to cgDBCzPA.

The element structures of the light-emitting element 1 and thecomparative light-emitting element 1 are shown in a table below.

TABLE 1 Hole-injection Hole-transport Light-emitting First electron-Second electron- layer layer layer transport layer transport layer 10 nm10 nm 25 nm 10 nm 15 nm Light-emitting HAT-CN PCBBiF cgDBCzPA:2mDBTBPDBq-II BPhen element 1 1,6mMemFLPAPrn Comparative (1:0.05)cgDBCzPA light-emitting element 1

The light-emitting element 1 and the comparative light-emitting element1 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied to surround the elements and UV treatmentand heat treatment at 80° C. for an hour were performed at the time ofsealing). Then, initial characteristics and reliability of theselight-emitting elements were measured. Note that the measurements werecarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 14 shows luminance-current density characteristics of thelight-emitting element 1 and the comparative light-emitting element 1.FIG. 15 shows current efficiency-luminance characteristics thereof. FIG.16 shows luminance-voltage characteristics thereof. FIG. 17 showscurrent-voltage characteristics thereof. FIG. 18 shows external quantumefficiency-luminance characteristics thereof. FIG. 19 shows emissionspectra thereof. Table 2 shows main characteristics of thelight-emitting elements at approximately 1000 cd/m².

TABLE 2 Current Current External quantum Voltage Current densityefficiency efficiency (V) (mA) (mA/cm²) Chromaticity x Chromaticity y(cd/A) (%) Light-emitting element 1 3.0 0.31 8 0.14 0.18 10.8 8.3Comparative light-emitting 2.9 0.34 9 0.14 0.18 10.9 8.4 element 1

It can be found from FIG. 14, FIG. 15, FIG. 16, FIG. 17, FIG. 18, FIG.19, and Table 2 that each of the light-emitting elements is a bluelight-emitting element with favorable characteristics.

FIG. 20 shows a change in luminance of the light-emitting element withdriving time under the conditions where the initial luminance was 5000cd/m² and the current density was constant. As shown in FIG. 20, adecrease in luminance with accumulation of driving time of thelight-emitting element 1 of one embodiment of the present invention issmaller than that of the comparative light-emitting element 1;therefore, the light-emitting element 1 has a long lifetime.

Furthermore, a decrease in luminance of the light-emitting element 1 inwhich HAT-CN, which is an organic acceptor, is used for thehole-injection layer is small in the high luminance region as shown inFIG. 15 and FIG. 18. Therefore, with the structure of the light-emittingelement 1 of this example, roll-off on the high luminance side can besuppressed, and the efficiency can be kept high even when thelight-emitting element 1 emits light with high luminance.

According to the cyclic voltammetry (CV) measurement results, the LUMOlevels of cgDBCzPA, 2mDBTBPDBq-II, and BPhen are estimated to −2.74 eV,−2.94 eV, and −2.63 eV, respectively. Therefore, the light-emittingelement 1 is one embodiment of the present invention.

Here, the HOMO level of PCBBiF is estimated to −5.36 eV from the CVmeasurement result, and this value is relatively high (higher than orequal to −5.4 eV); therefore, electron extraction by HAT-CN iseffectively performed. However, since the HOMO level of cgDBCzPA, whichis an anthracene derivative, used for the light-emitting layer isestimated to −5.69 eV from the CV measurement result, the injectionbarrier from the hole-transport layer to the host material in thelight-emitting layer is as large as 0.33 eV in each of thelight-emitting element 1 and the comparative light-emitting element 1.For this reason, the comparative light-emitting element 1 has excessiveelectrons, which cause a large roll-off and a decrease in lifetime. Incontrast, in the light-emitting element 1, such low hole-injectioncapability due to the organic acceptor can be compensated by oneembodiment of the present invention; thus, small roll-off and a longlifetime are achieved in the light-emitting element 1.

Example 2

In this example, a light-emitting element 2 of one embodiment of thepresent invention, which corresponds to the light-emitting elementdescribed in Embodiment 1, and a comparative light-emitting element 2which has a different structure from the light-emitting element of oneembodiment of the present invention were compared, and the results aredescribed. A difference between the light-emitting element 2 and thecomparative light-emitting element 2 is only a material for the firstelectron-transport layer. Structural formulae of organic compounds usedin the light-emitting element 2 and the comparative light-emittingelement 2 are shown below.

(Method of Fabricating Light-Emitting Element 2)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the anode101 was formed. The thickness thereof was 110 nm and the electrode areawas 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for an hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Next, the substrate provided with the anode 101 was fixed to a substrateholder provided in the vacuum evaporation device such that the side onwhich the anode 101 was formed faced downward. After the pressure waslowered to approximately 10⁻⁴ Pa,2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN) represented by the structural formula (i) was deposited to athickness of 10 nm over the anode 101 by an evaporation method usingresistance heating, whereby the hole-injection layer 111 was formed.

Next, over the hole-injection layer 111,N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula (ii)was deposited to a thickness of 10 nm to form the hole-transport layer112.

Furthermore, over the hole-transport layer 112, the light-emitting layer113 was formed by co-evaporation of7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by the above structural formula(iii) and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by the above structuralformula (iv) in a weight ratio of 1:0.05 (=cgDBCzPA: 1,6mMemFLPAPrn) toa thickness of 25 nm.

After that,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-H) represented by the structural formula (v)was deposited to a thickness of 10 nm as the first electron-transportlayer 114-1 over the light-emitting layer 113, and then2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen) represented by the structural formula (vii) was deposited to athickness of 15 nm as the second electron-transport layer 114-2.

After the formation of the first electron-transport layer 114-1 and thesecond electron-transport layer 114-2, lithium fluoride (LiF) wasdeposited by evaporation to a thickness of 1 nm and aluminum wasdeposited by evaporation to a thickness of 200 nm to form the cathode102. Thus, the light-emitting element 2 in this example was fabricated.

(Method for Fabricating Comparative Light-Emitting Element 2)

The comparative light-emitting element 2 was fabricated in the samemanner as the light-emitting element 2 except for changing 2mDBTBPDBq-IIin the first electron-transport layer to cgDBCzPA.

The element structures of the light-emitting element 2 and thecomparative light-emitting element 2 are shown in a table below.

TABLE 3 Hole-injection Hole-transport Light-emitting First electron-Second electron- layer layer layer transport layer transport layer 10 nm10 nm 25 nm 10 nm 15 nm Light-emitting HAT-CN PCBBiF cgDBCzPA:2mDBTBPDBq-II NBPhen element 2 1,6mMemFLPAPrn Comparative (1:0.05)cgDBCzPA light-emitting element 2

The light-emitting element 2 and the comparative light-emitting element2 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied to surround the elements and UV treatmentand heat treatment at 80° C. for an hour were performed at the time ofsealing). Then, initial characteristics and reliability of theselight-emitting elements were measured. Note that the measurements werecarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 21 shows luminance-current density characteristics of thelight-emitting element 2 and the comparative light-emitting element 2.FIG. 22 shows current efficiency-luminance characteristics thereof. FIG.23 shows luminance-voltage characteristics thereof. FIG. 24 showscurrent-voltage characteristics thereof. FIG. 25 shows external quantumefficiency-luminance characteristics thereof. FIG. 26 shows emissionspectra thereof. Table 4 shows main characteristics of thelight-emitting elements at approximately 1000 cd/m².

TABLE 4 Current External quantum Voltage Current Current densityefficiency efficiency (V) (mA) (mA/cm²) Chromaticity x Chromaticity y(cd/A) (%) Light- 3.2 0.48 12 0.14 0.18 10.9 8.4 emitting element 2Comparative 3.0 0.40 10 0.14 0.18 11.2 8.6 light-emitting element 2

It can be found from FIG. 21, FIG. 22, FIG. 23, FIG. 24, FIG. 25, FIG.26, and Table 4 that each of the light-emitting elements is a bluelight-emitting element with favorable characteristics.

FIG. 27 shows a change in luminance of the light-emitting element withdriving time under the conditions where the initial luminance was 5000cd/m² and the current density was constant. As shown in FIG. 27, adecrease in luminance with accumulation of driving time of thelight-emitting element 2 of one embodiment of the present invention issmaller than that of the comparative light-emitting element 2;therefore, the light-emitting element 2 has a long lifetime.

Furthermore, a decrease in luminance of the light-emitting element 2 inwhich HAT-CN, which is an organic acceptor, is used for thehole-injection layer is small in the high luminance region as shown inFIG. 22 and FIG. 25. Therefore, with the structure of the light-emittingelement 2 of this example, roll-off on the high luminance side can besuppressed, and the efficiency can be kept high even when thelight-emitting element 2 emits light with high luminance.

According to the cyclic voltammetry (CV) measurement results, the LUMOlevels of cgDBCzPA, 2mDBTBPDBq-II, and NBPhen are estimated to −2.74 eV,−2.94 eV, and −2.83 eV, respectively. Therefore, the light-emittingelement 2 is one embodiment of the present invention.

Here, the HOMO level of PCBBiF is estimated to −5.36 eV from the CVmeasurement result, and this value is relatively high (higher than orequal to −5.4 eV); therefore, electron extraction by HAT-CN iseffectively performed. However, since the HOMO level of cgDBCzPA, whichis an anthracene derivative, used for the light-emitting layer isestimated to −5.69 eV from the CV measurement result, the injectionbarrier from the hole-transport layer to the host material in thelight-emitting layer is as large as 0.33 eV in each of thelight-emitting element 2 and the comparative light-emitting element 2.For this reason, the comparative light-emitting element 2 has excessiveelectrons, which cause a large roll-off and a decrease in lifetime. Incontrast, in the light-emitting element 2, such low hole-injectioncapability due to the organic acceptor can be compensated by oneembodiment of the present invention; thus, small roll-off and a longlifetime are achieved in the light-emitting element 2.

Example 3

In this example, a light-emitting element 3 of one embodiment of thepresent invention, which corresponds to the light-emitting elementdescribed in Embodiment 1, and a comparative light-emitting element 3which has a different structure from the light-emitting element of oneembodiment of the present invention were compared, and the results aredescribed. A difference between the light-emitting element 3 and thecomparative light-emitting element 3 is only a material for the firstelectron-transport layer. Structural formulae of organic compounds usedin the light-emitting element 3 and the comparative light-emittingelement 3 are shown below.

(Method of Fabricating Light-Emitting Element 3)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the anode101 was formed. The thickness thereof was 110 nm and the electrode areawas 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for an hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Next, the substrate provided with the anode 101 was fixed to a substrateholder provided in the vacuum evaporation device such that the side onwhich the anode 101 was formed faced downward. After the pressure waslowered to approximately 10⁻⁴ Pa,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)represented by the structural formula (viii) and molybdenum(VI) oxidewere co-deposited over the anode 101 by an evaporation method usingresistance heating, whereby the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was 10 nm. The weight ratio ofPCPPn to molybdenum oxide was adjusted to be 4:2 (=PCPPn: molybdenumoxide).

Next, PCPPn was deposited to a thickness of 20 nm over thehole-injection layer 111 to form the hole-transport layer 112.

Furthermore, over the hole-transport layer 112, the light-emitting layer113 was formed by co-evaporation of7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by the above structural formula(iii) and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPm) represented by the above structuralformula (iv) in a weight ratio of 1:0.03 (=cgDBCzPA: 1,6mMemFLPAPrn) toa thickness of 25 nm.

After that,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by the structural formula (v)was deposited to a thickness of 10 nm as the first electron-transportlayer 114-1 over the light-emitting layer 113, and then2,2′-(pyridine-2,6-diyl)bis(4,6-diphenylpyrimidine) (abbreviation:2,6(P2Pm)2Py) represented by the structural formula (ix) was depositedto a thickness of 15 nm as the second electron-transport layer 114-2.

After the formation of the first electron-transport layer 114-1 and thesecond electron-transport layer 114-2, lithium fluoride (LiF) wasdeposited by evaporation to a thickness of 1 nm and aluminum wasdeposited by evaporation to a thickness of 200 nm to form the cathode102. Thus, the light-emitting element 3 in this example was fabricated.

(Method for Fabricating Comparative Light-Emitting Element 3)

The comparative light-emitting element 3 was fabricated in the samemanner as the light-emitting element 3 except for changing 2mDBTBPDBq-IIin the first electron-transport layer to cgDBCzPA.

The element structures of the light-emitting element 3 and thecomparative light-emitting element 3 are shown in a table below.

TABLE 5 Hole-injection Hole-transport Light-emitting First electron-Second electron- layer layer layer transport layer transport layer 10 nm20 nm 25 nm 10 nm 15 nm Light-emitting PCPPn:MoOx PCPPn cgDBCzPA:2mDBTBPDBq-II 2,6(P2Pm)2Py element 3 (4:2) 1,6mMemFLPAPrn Comparative(1:0.03) cgDBCzPA light-emitting element 3

The light-emitting element 3 and the comparative light-emitting element3 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied to surround the elements and UV treatmentand heat treatment at 80° C. for an hour were performed at the time ofsealing). Then, initial characteristics and reliability of theselight-emitting elements were measured. Note that the measurements werecarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 28 shows luminance-current density characteristics of thelight-emitting element 3 and the comparative light-emitting element 3.FIG. 29 shows current efficiency-luminance characteristics thereof. FIG.30 shows luminance-voltage characteristics thereof. FIG. 31 showscurrent-voltage characteristics thereof. FIG. 32 shows external quantumefficiency-luminance characteristics thereof. FIG. 33 shows emissionspectra thereof. Table 6 shows main characteristics of thelight-emitting elements at approximately 1000 cd/m².

TABLE 6 External Current Current quantum Voltage Current densityefficiency efficiency (V) (mA) (mA/cm²) Chromaticity x Chromaticity y(cd/A) (%) Light-emitting 3.2 0.38 10 0.14 0.15 13.7 11.8 element 3Comparative light- 3.0 0.29 7 0.14 0.15 13.1 11.2 emitting element 3

It can be found from FIG. 28, FIG. 29, FIG. 30, FIG. 31, FIG. 32, FIG.33, and Table 6 that each of the light-emitting elements is a bluelight-emitting element with favorable characteristics, and particularly,the light-emitting element 3 shows an extremely high external quantumefficiency exceeding 12% (under assumption of Lambertian distribution).

FIG. 34 shows a change in luminance of the light-emitting element withdriving time under the conditions where the initial luminance was 5000cd/m² and the current density was constant. As shown in FIG. 34, adecrease in luminance with accumulation of driving time of thelight-emitting element 3 of one embodiment of the present invention issmaller than that of the comparative light-emitting element 3;therefore, the light-emitting element 1 has a long lifetime.

According to the cyclic voltammetry (CV) measurement results, the LUMOlevels of cgDBCzPA, 2mDBTBPDBq-II, and 2,6(P2Pm)2Py are estimated to−2.74 eV, −2.94 eV, and −2.78 eV, respectively. Therefore, thelight-emitting element 3 is one embodiment of the present invention.

In the light-emitting element 3, a composite material of ahole-transport material and an acceptor material is used for thehole-injection layer. The hole-transport material used for thehole-injection layer is PCPPn, which is also used for the hole-transportlayer, and the HOMO level of PCPPn is as deep as −5.80 eV by the CVmeasurement. Accordingly, PCPPn has a favorable hole-injection propertywith respect to cgDBCzPA (HOMO level: −5.69 eV) which is the hostmaterial in the light-emitting layer, but in general, hole injectionfrom the anode is difficult. However, since a transition metal oxide isused as the acceptor material in the light-emitting element 3, theacceptor material shows an acceptor property with respect to ahole-transport material having a HOMO level lower (deeper) than −5.4 eV(the acceptor material can extract an electron by at least applicationof an electric field). Consequently, hole injection and hole transportfrom the anode to the hole-injection layer, the hole-transport layer,and the light-emitting layer are smoothly performed. By the combinationof this hole-injection layer and one embodiment of the presentinvention, the light-emitting element 3 achieves not only low-voltagedriving but also an unexpectedly high external quantum efficiency and along lifetime, which are major characteristics.

Example 4

In this example, a light-emitting element 4 of one embodiment of thepresent invention, which corresponds to the light-emitting elementdescribed in Embodiment 1, and a comparative light-emitting element 4which has a different structure from the light-emitting element of oneembodiment of the present invention were compared, and the results aredescribed. A difference between the light-emitting element 4 and thecomparative light-emitting element 4 is only a material for the firstelectron-transport layer. Structural formulae of organic compounds usedin the light-emitting element 4 and the comparative light-emittingelement 4 are shown below.

(Method of Fabricating Light-Emitting Element 4)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the anode101 was formed. The thickness thereof was 110 nm and the electrode areawas 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for an hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Next, the substrate provided with the anode 101 was fixed to a substrateholder provided in the vacuum evaporation device such that the side onwhich the anode 101 was formed faced downward. After the pressure waslowered to approximately 10-Pa,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)represented by the structural formula (viii) and molybdenum(VI) oxidewere co-deposited over the anode 101 by an evaporation method usingresistance heating, whereby the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was 10 nm. The weight ratio ofPCPPn to molybdenum oxide was adjusted to be 4:2 (=PCPPn: molybdenumoxide).

Next, PCPPn was deposited to a thickness of 30 nm over thehole-injection layer 111 to form the hole-transport layer 112.

Furthermore, over the hole-transport layer 112, the light-emitting layer113 was formed by co-evaporation of7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by the above structural formula(iii) and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by the above structuralformula (iv) in a weight ratio of 1:0.03 (=cgDBCzPA: 1,6mMemFLPAPrn) toa thickness of 25 nm.

After that,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-U) represented by the structural formula (v)was deposited to a thickness of 10 nm as the first electron-transportlayer 114-1 over the light-emitting layer 113, and then2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen) represented by the structural formula (vii) was deposited to athickness of 15 nm as the second electron-transport layer 114-2.

After the formation of the first electron-transport layer 114-1 and thesecond electron-transport layer 114-2, lithium fluoride (LiF) wasdeposited by evaporation to a thickness of 1 nm and aluminum wasdeposited by evaporation to a thickness of 200 nm to form the cathode102. Thus, the light-emitting element 4 in this example was fabricated.

(Method for Fabricating Comparative Light-Emitting Element 4)

The comparative light-emitting element 4 was fabricated in the samemanner as the light-emitting element 4 except for changing 2mDBTBPDBq-IIin the first electron-transport layer to cgDBCzPA.

The element structures of the light-emitting element 4 and thecomparative light-emitting element 4 are shown in a table below.

TABLE 7 Hole-injection Hole-transport Light-emitting First electron-Second electron- layer layer layer transport layer transport layer 10 nm30 nm 25 nm 10 nm 15 nm Light-emitting PCPPn:MoOx PCPPn cgDBCzPA:2mDBTBPDBq-II NBPhen element 4 (4:2) 1,6mMemFLPAPrn Comparative (1:0.03)cgDBCzPA light-emitting element 4

The light-emitting element 4 and the comparative light-emitting element4 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied to surround the elements and UV treatmentand heat treatment at 80° C. for an hour were performed at the time ofsealing). Then, initial characteristics and reliability of theselight-emitting elements were measured. Note that the measurements werecarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 35 shows luminance-current density characteristics of thelight-emitting element 4 and the comparative light-emitting element 4.FIG. 36 shows current efficiency-luminance characteristics thereof. FIG.37 shows luminance-voltage characteristics thereof. FIG. 38 showscurrent-voltage characteristics thereof. FIG. 39 shows external quantumefficiency-luminance characteristics thereof. FIG. 40 shows emissionspectra thereof. Table 8 shows main characteristics of thelight-emitting elements at approximately 1000 cd/m².

TABLE 8 External Current Current quantum Voltage Current densityefficiency efficiency (V) (mA) (mA/cm²) Chromaticity x Chromaticity y(cd/A) (%) Light-emitting 3.3 0.27 7 0.14 0.18 16.9 13.7 element 4Comparative light- 3.2 0.23 6 0.14 0.18 16.2 13.1 emitting element 4

It can be found from FIG. 35, FIG. 36, FIG. 37, FIG. 38, FIG. 39, FIG.40, and Table 8 that each of the light-emitting elements is a bluelight-emitting element with favorable characteristics, and particularly,the light-emitting element 4 shows an extremely high external quantumefficiency exceeding 13.7% (under assumption of Lambertiandistribution).

FIG. 41 shows a change in luminance of the light-emitting element withdriving time under the conditions where the initial luminance was 5000cd/m² and the current density was constant. As shown in FIG. 41, adecrease in luminance with accumulation of driving time of thelight-emitting element 4 of one embodiment of the present invention issmaller than that of the comparative light-emitting element 4;therefore, the light-emitting element 4 has a long lifetime.

According to the cyclic voltammetry (CV) measurement results, the LUMOlevels of cgDBCzPA, 2mDBTBPDBq-II, and NBPhen are estimated to −2.74 eV,−2.94 eV, and −2.83 eV, respectively. Therefore, the light-emittingelement 4 is one embodiment of the present invention.

In the light-emitting element 4, a composite material of ahole-transport material and an acceptor material is used for thehole-injection layer. The hole-transport material used for thehole-injection layer is PCPPn, which is also used for the hole-transportlayer, and the HOMO level of PCPPn is as deep as −5.80 eV by the CVmeasurement. Accordingly, PCPPn has a favorable hole-injection propertywith respect to cgDBCzPA (HOMO level: −5.69 eV) which is the hostmaterial in the light-emitting layer, but in general, hole injectionfrom the anode is difficult. However, since a transition metal oxide isused as the acceptor material in the light-emitting element 4, theacceptor material shows an acceptor property with respect to ahole-transport material having a HOMO level lower (deeper) than −5.4 eV(the acceptor material can extract an electron by at least applicationof an electric field). Consequently, hole injection and hole transportfrom the anode to the hole-injection layer, the hole-transport layer,and the light-emitting layer are smoothly performed. By the combinationof this hole-injection layer and one embodiment of the presentinvention, the light-emitting element 4 achieves not only low-voltagedriving but also an unexpectedly high external quantum efficiency and along lifetime, which are major characteristics.

Example 5

Compounds that can be used for the light-emitting element of oneembodiment of the present invention are listed in tables. Compoundsincluding a condensed aromatic ring skeleton including 3 to 6 ringswhich are suitably used as the host material in the light-emitting layerare listed as No. 1 to No. 13 in Table 9. In addition, compounds eachhaving a low LUMO level which are suitably used for the firstelectron-transport layer are listed as No. 14 to No. 35 in Table 10;compounds suitably used for the second electron-transport layer (whoseLUMO levels tend to be relatively higher than the LUMO levels of thecompounds suitably used for the first electron-transport layer) arelisted as No. 36 to No. 46 in Table 11. The molecular structure of thecompounds are also shown below. In the tables, EmL refers to thelight-emitting layer, ETL1 refers to the first electron-transport layer,and ETL2 refers to the second electron-transport layer. The LUMO levelswere obtained through a cyclic voltammetry (CV) measurement to bedescribed later.

TABLE 9 LUMO (eV) measured No. Abbreviation Layer Skeleton by CV 1 CzPAEmL anthracene −2.73 2 cgDBCzPA EmL anthracene −2.74 3 PCzPA EmLanthracene −2.70 4 2mDBFPPA-II EmL anthracene −2.78 5 t-BuDNA EmLanthracene −2.68 6 BH-1 EmL anthracene −2.73 7 DPT EmL tetracene −3.05 8rubrene EmL tetracene −3.11 9 TBRb EmL tetracene −2.99 10 TPB3 EmLpyrene −2.64 11 BPPF EmL pyrene −2.58 12 spiro-pye EmL pyrene −2.62 13TBP EmL perylene −2.77

TABLE 10 LUMO (eV) measured No. Abbreviation Layer Skeleton by CV 142mDBTPDBq-II ETL1 dibenzoquinoxaline −2.95 15 2mDBTBPDBq-II ETL1dibenzoquinoxaline −2.94 16 2mDBTBPDBq-III ETL1 dibenzoquinoxaline −2.9417 2mDBTBPDBq-IV ETL1 dibenzoquinoxaline −2.94 18 PCPDBq ETL1dibenzoquinoxaline −2.93 19 2mCzPDBq-III ETL1 dibenzoquinoxaline −2.9920 2mCzBPDBq ETL1 dibenzoquinoxaline −2.95 21 7mDBTPDBq-II ETL1dibenzoquinoxaline −2.86 22 7mDBTBPDBq-II ETL1 dibenzoquinoxaline −2.8623 2,6(P-Bpn)2Py ETL1 dibenzoquinazoline −2.92 24 4mDBTBPBfpm-II ETL1pyrimidine (fused) −2.96 25 4mCzBPBfpm ETL1 pyrimidine (fused) −2.97 264,6mDBTP2Pm-II ETL1 pyrimidine −2.83 27 4,6mCzP2Pm ETL1 pyrimidine −2.8828 B3PYMPM ETL1 pyrimidine −2.84 29 2,6(P2Pm)2Py ETL1 pyrimidine −2.7830 PPDN ETL1 pyrazine (fused) −3.83 31 2PYPR ETL1 pyrazine (fused) −3.2232 3PYPR ETL1 pyrazine (fused) −3.26 33 2Py3Tzn ETL1 triazine −3.15 34TmPPPyTz ETL1 triazine −3.00 35 CPCBPTz ETL1 triazine −2.99

TABLE 11 LUMO (eV) measured No. Abbreviation Layer Skeleton by CV 36 BCPETL2 phenanthroline −2.55 37 BPhen ETL2 phenanthroline −2.63 38 NBPhenETL2 phenanthroline −2.83 39 Phen2BP ETL2 phenanthroline −2.79 404,4′mCzP2BPy ETL2 bipyridine −2.66 41 4,4′mDBTP2BPy-II ETL2 bipyridine−2.63 42 4,4′DBfP2BPy ETL2 bipyridine −2.60 43 3TPYMB ETL2 pyridine−2.63 44 TmPyPB ETL2 pyridine −2.23 45 BP4mPy ETL2 pyridine −2.26 46BmPyPhB ETL2 pyridine −2.29

It is found that, for example, the LUMO levels of compounds eachincluding an anthracene skeleton are approximately −2.7 eV from thetable. In the case where the compound including an anthracene skeletonis used as the host material in the light-emitting layer, a compoundwhose LUMO level is lower than the LUMO level of the compound includingan anthracene skeleton may be used for the first electron-transportlayer, and any of the compounds of No. 14 to No. 35 can be used.However, since an energy difference between the LUMO levels ispreferably lower than or equal to 0.3 eV, a compound including adibenzoquinoxaline skeleton, a dibenzoquinazoline skeleton, or apyrimidine skeleton is particularly preferred because the LUMO level isapproximately −2.80 eV to −2.99 eV in many cases. In that case, with useof any of the compounds of No. 36 to No. 46 for the secondelectron-transport layer, the light-emitting element of one embodimentof the present invention can be fabricated.

In the case where a compound including another skeleton is used as thehost material in the light-emitting layer, the light-emitting element ofone embodiment of the present invention can be fabricated in a similarmanner. For example, in the case where a compound including a tetraceneskeleton is used as the host material in the light-emitting layer, notall the compounds of No. 14 to No. 36 are suitably used for the firstelectron-transport layer because the LUMO level of the compoundincluding a tetracene skeleton is relatively low. However, with use of,for example, a fused pyrazine compound or a fused triazine compound forthe first electron-transport layer, the light-emitting element of oneembodiment of the present invention can be fabricated.

Note that the compounds including a phenanthroline skeleton, abipyridine skeleton, or a pyridine skeleton are suitably used for thesecond electron-transport layer because of their relatively high LUMOlevels among the compounds having an electron-transport property asshown in the table. Moreover, in the case where any of these compoundsis used in contact with the cathode, the electron-injection propertyfrom the cathode is favorable, which is another reason that thesecompounds are suitably used for the second electron-transport layer.

Here, cyclic voltammetry (CV) measurement is described. Anelectrochemical analyzer (ALS model 600A or 600C, manufactured by BASInc.) was used for the CV measurement.

As for a solution used for the CV measurement, dehydrateddimethylformamide (DMF, produced by Sigma-Aldrich Inc., 99.8%, CatalogNo. 227056-12) was used as a solvent, and tetra-n-butylammoniumperchlorate (electrochemical grade, Wako Pure Chemical Industries, Ltd.,manufacturer's code: 043999, CAS. No. 1923-70-2), which was a supportingelectrolyte, was dissolved in the solvent so that the concentration oftetra-n-butylammonium perchlorate can be 100 mmol/L. Further, themeasurement target was dissolved in the solution so that theconcentration thereof can be 2 mmol/L. Then, the solution was put intoan electrochemical cell, electrodes were set, and then degasification byargon bubbling was performed for approximately 30 minutes. Theelectrodes used for the measurement were a platinum electrode (producedby BAS Inc., PTE platinum electrode) as a working electrode, a platinumelectrode (produced by BAS Inc., Pt counter electrode) as an auxiliaryelectrode, and a reference electrode for nonaqueous solvent (produced byBAS Inc., RE-7 reference electrode for nonaqueous solvent (Ag/Ag⁺)) as areference electrode. In the CV measurement, room temperature (20° C. to25° C.) and a scan rate of 0.1 V/sec were employed. Note that thepotential energy of the reference electrode with respect to the vacuumlevel was assumed to be −4.94 eV in this example.

The LUMO level (reduction potential) and the HOMO level (oxidationpotential) were obtained from the CV measurement results. From anoxidation peak potential (from the reduction state to the neutral state)E_(pc) [V] and a reduction peak potential (from the neutral state to thereduction state) E_(pa) [V], a half wave potential (a potentialintermediate between E_(pa) and E_(pc)) was calculated(=(E_(pa)+E_(pc))/2 [V]). Then, this half wave potential was subtractedfrom the potential energy of the reference electrode with respect to thevacuum level (−4.94 eV), so that the LUMO level was obtained. From anoxidation peak potential (from the neutral state to the oxidation state)E_(pa) [V] and a reduction peak potential (from the oxidation state tothe neutral state) E_(pc) [V], the half wave potential (the potentialintermediate between E_(pa) and E_(pc)) was calculated(=(E_(pa)+E_(pc))/2 [V]). Then, this half wave potential was subtractedfrom the potential energy of the reference electrode with respect to thevacuum level (−4.94 eV), so that the HOMO level was obtained.

This application is based on Japanese Patent Application serial no.2015-099866 filed with Japan Patent Office on May 15, 2015 and JapanesePatent Application serial no. 2016-049620 filed with Japan Patent Officeon Mar. 14, 2016, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A light-emitting element comprising: an anode; acathode; and an EL layer between the anode and the cathode, wherein theEL layer comprises a light-emitting layer, a first electron-transportlayer, and a second electron-transport layer, wherein the firstelectron-transport layer is between the light-emitting layer and thesecond electron-transport layer, wherein the light-emitting layer has aregion in contact with the first electron-transport layer, wherein thesecond electron-transport layer has a region in contact with the firstelectron-transport layer, wherein the light-emitting layer comprises afluorescent substance and a host material, wherein the firstelectron-transport layer comprises a first material, wherein the secondelectron-transport layer comprises a second material, wherein a LUMOlevel of the host material is higher than a LUMO level of the firstmaterial, wherein a LUMO level of the second material is higher than theLUMO level of the first material, wherein the host material is asubstance including a condensed aromatic ring skeleton including 3 to 6rings, wherein the first material is a substance including a firstheteroaromatic ring skeleton, wherein the second material is a substanceincluding a second heteroaromatic ring skeleton, and wherein thesubstance including the first heteroaromatic ring skeleton is differentfrom the substance including the second heteroaromatic ring skeleton. 2.The light-emitting element according to claim 1, wherein each of thesubstance including the first heteroaromatic ring skeleton and thesubstance including the second heteroaromatic ring skeleton is asubstance including a six-membered nitrogen-containing heteroaromaticring skeleton.
 3. The light-emitting element according to claim 1,wherein the substance including the first heteroaromatic ring skeletonis a substance including a condensed heteroaromatic ring skeleton. 4.The light-emitting element according to claim 1, wherein the substanceincluding the first heteroaromatic ring skeleton is a substanceincluding a condensed heteroaromatic ring skeleton including a diazineskeleton or a triazine skeleton.
 5. The light-emitting element accordingto claim 1, wherein the substance including the first heteroaromaticring skeleton is a substance including a pyrazine skeleton or apyrimidine skeleton.
 6. The light-emitting element according to claim 1,wherein the substance including the first heteroaromatic ring skeletonis a substance including a dibenzoquinoxaline skeleton.
 7. Thelight-emitting element according to claim 1, wherein the host materialis a substance including an anthracene skeleton.
 8. The light-emittingelement according to claim 1, wherein the second electron-transportlayer is in contact with the cathode.
 9. The light-emitting elementaccording to claim 1, wherein the EL layer further comprises ahole-injection layer, wherein the hole-injection layer is in contactwith the anode, and wherein the hole-injection layer comprises anorganic acceptor material.
 10. The light-emitting element according toclaim 9, wherein the organic acceptor material is2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene.
 11. Thelight-emitting element according to claim 1, wherein the fluorescentsubstance exhibits blue light.
 12. A light-emitting device comprising:the light-emitting element according to claim 1; and at least one of atransistor and a substrate.
 13. An electronic device comprising: thelight-emitting device according to claim 12; and at least one of asensor, an operation button, a speaker, and a microphone.
 14. A lightingdevice comprising: the light-emitting device according to claim 12; anda housing.